The biggest Election Day question haunting the White House is this: did President Obama blow it by bowing to Southern senate Democrats and delaying action on effectively legalizing millions of undocumented workers?

Mr. Obama promised to use executive authority before the election to defer deportation of untold millions of undocumented laborers with no felony convictions and a long record of residency. The administration never said how many would qualify but estimates ranged from three million to nine million.

Under pressure from Sens. David Pryor of Arkansas, Mary Landrieu of Louisiana and Kay Hagan of North Carolina, who all preemptively opposed the president’s use of executive authority, Mr. Obama decided to delay action until after the election on a flight back to Washington from the NATO summit in Wales.

After Mr. Obama delayed, Democratic Sens. Kay Hagan, of North Carolina and Jeanne Shaheen of New Hampshire broke with the White House and backed Pryor and Landrieu on a symbolic amendment to derail any Obama moves on immigration. These Democrats all hoped to nullify immigration by publicly opposing Obama. But the issue has persisted and Pryor, Landrieu and Shaheen have struggled to credibly distance themselves from Obama–beyond asking him to stay away from their campaign rallies.

Colorado is a different story. Democratic Sen. Mark Udall is trailing, and the Latino vote could be decisive. With tight races for Hagan and Rep. Bruce Braley in Iowa and Michelle Nunn in Georgia, states with small but rising percentages of Democratic-leaning Latino voters, the festering resentment over Mr. Obama’s inaction may keep those voters at home when the president and the Democratic Senate majority he craves need them the most.

“He demobilized Latino activists across the country,” said Gary Segura, partner of the firm Latino Decisions, which polls Latinos extensively and monitors election turnout. Segura sees depressed Latino turnout figuring prominently in these Senate races and the nail-biter governor’s race in Florida pitting Republican-turned Democrat Charlie Crist against GOP Gov. Rick Scott.

“If Nunn pulls an upset and Hagan and Udall win, Obama looks like a freaking genius but if Crist or Udall lose, and Hagan loses, and Nunn loses, then he looks like he delayed for nothing, and the delay may have cost Democrats enthusiastic support of Latino voters,” Segura said.

• Do Latino voters matter in the midterm election?
• Full coverage: Campaign 2014

Senior administration officials say the delay was “a tough call,” and the downstream political effect was hotly debated at the time. Obama expected some blowback from Latino activists but decided Democrats would have a better chance of making their re-election case without the distraction of a polarizing executive gesture on immigration.

But senior GOP Senate aides now believe Obama and Democrats would have been better off with the rallying cry of action on immigration to motivate Latinos and white liberal Democrats energized by a fight with civil rights overtones.

Republicans point to Scott Brown’s ability in New Hampshire, a state with few Latino voters and no proximity to southern immigration woes, to use anxiety about border security to pull within striking distance of Shaheen.

The upshot: Republicans are reaping most, if not all, of the voter unease about border security, immigration and wage competition. Meanwhile, Democrats are enjoying little or none of the grassroots mobilization that would have come with Obama keeping his promise to shield millions of adult immigrants from deportation.

“It may have been the biggest strategic mistake of this cycle,” said a top Senate Republican strategist. “There is no proof it helped Pryor or Landrieu, and it allowed us to dodge a debate over impeachment or some of the distractions that would have arisen had Obama acted.”

An Obama executive order could have provoked some Republicans, like Rep. Steve King of Iowa, to call for impeachment or block funding for Obama’s immigration move–something that might have altered the debate in Iowa’s Senate race.

The same might have applied in North Carolina, where Hagan spent the summer running against what she called the harsh legislative actions led by her opponent, House Speaker Thom Tillis. Udall, who backed executive action, might have leveraged the immigration debate against GOP Rep. Corey Gardner, broadening his agenda beyond what supporters regard as a one-note obsession with women’s reproductive issues.

“The Democrats got very little from delaying the executive action decision,” said Frank Sharry, executive director of America’s Voice. “They lost an opportunity to mobilize base voters, to send Republicans into crazy land and to consolidate the fast-growing Latino community as a core constituency in the Democratic coalition.”

Mr. Obama’s problems were on vivid and grating display Sunday in Bridgeport, Conn., when Latino activists interrupted his rep-election pitch for embattled Gov. Dannel Malloy six times. Latino protesters have heckled Obama frequently during his few campaign stops this cycle, routinely picket outside the White House and have also disrupted fund-raisers.

At one point Sunday, Obama told the hecklers “Shame on you.” Later, Obama told the hecklers they didn’t understand the basics.

“Hold on a second, everybody,” Obama said. “Quiet down. It’s a choice that we’ve got to make between two very different visions of America. I am sympathetic to those who are concerned about immigration. That’s why we fought for immigration reform. It’s the other party that’s blocked it. Unfortunately, folks get frustrated and so they want to yell at everybody.”

House Republicans refused to bring any immigration reform bills to the floor, killing the issue and prompting Obama to investigate and then promise to legalize millions without congressional consent. What’s now gnawing at the White House is the realization that failing to act may not save vulnerable southern Senate Democrats and may jeopardize other Democrats central to maintaining a Democratic majority.

As for Obama’s post-election plans, White House Press Secretary Josh Earnest refused to discuss any agenda items or deliberations on dealing with a possible GOP majority. He did make one promise, though.

“The one thing we know that’s going to happen before the end of the year is the president’s going to take action to use his executive authority to fix those aspects of our broken immigration system that can be fixed using executive authority,” Earnest said.

It’s a promise Latino activists have heard before. For Senate Democrats and the White House that cherishes its current majority, it may arrive too late.

The biggest Election Day question haunting the White House is this: did President Obama blow it by bowing to Southern senate Democrats and delaying action on effectively legalizing millions of undocumented workers?

Mr. Obama promised to use executive authority before the election to defer deportation of untold millions of undocumented laborers with no felony convictions and a long record of residency. The administration never said how many would qualify but estimates ranged from three million to nine million.

Under pressure from Sens. David Pryor of Arkansas, Mary Landrieu of Louisiana and Kay Hagan of North Carolina, who all preemptively opposed the president’s use of executive authority, Mr. Obama decided to delay action until after the election on a flight back to Washington from the NATO summit in Wales.

After Mr. Obama delayed, Democratic Sens. Kay Hagan, of North Carolina and Jeanne Shaheen of New Hampshire broke with the White House and backed Pryor and Landrieu on a symbolic amendment to derail any Obama moves on immigration. These Democrats all hoped to nullify immigration by publicly opposing Obama. But the issue has persisted and Pryor, Landrieu and Shaheen have struggled to credibly distance themselves from Obama–beyond asking him to stay away from their campaign rallies.

Colorado is a different story. Democratic Sen. Mark Udall is trailing, and the Latino vote could be decisive. With tight races for Hagan and Rep. Bruce Braley in Iowa and Michelle Nunn in Georgia, states with small but rising percentages of Democratic-leaning Latino voters, the festering resentment over Mr. Obama’s inaction may keep those voters at home when the president and the Democratic Senate majority he craves need them the most.

“He demobilized Latino activists across the country,” said Gary Segura, partner of the firm Latino Decisions, which polls Latinos extensively and monitors election turnout. Segura sees depressed Latino turnout figuring prominently in these Senate races and the nail-biter governor’s race in Florida pitting Republican-turned Democrat Charlie Crist against GOP Gov. Rick Scott.

“If Nunn pulls an upset and Hagan and Udall win, Obama looks like a freaking genius but if Crist or Udall lose, and Hagan loses, and Nunn loses, then he looks like he delayed for nothing, and the delay may have cost Democrats enthusiastic support of Latino voters,” Segura said.

• Do Latino voters matter in the midterm election?
• Full coverage: Campaign 2014

Senior administration officials say the delay was “a tough call,” and the downstream political effect was hotly debated at the time. Obama expected some blowback from Latino activists but decided Democrats would have a better chance of making their re-election case without the distraction of a polarizing executive gesture on immigration.

But senior GOP Senate aides now believe Obama and Democrats would have been better off with the rallying cry of action on immigration to motivate Latinos and white liberal Democrats energized by a fight with civil rights overtones.

Republicans point to Scott Brown’s ability in New Hampshire, a state with few Latino voters and no proximity to southern immigration woes, to use anxiety about border security to pull within striking distance of Shaheen.

The upshot: Republicans are reaping most, if not all, of the voter unease about border security, immigration and wage competition. Meanwhile, Democrats are enjoying little or none of the grassroots mobilization that would have come with Obama keeping his promise to shield millions of adult immigrants from deportation.

“It may have been the biggest strategic mistake of this cycle,” said a top Senate Republican strategist. “There is no proof it helped Pryor or Landrieu, and it allowed us to dodge a debate over impeachment or some of the distractions that would have arisen had Obama acted.”

An Obama executive order could have provoked some Republicans, like Rep. Steve King of Iowa, to call for impeachment or block funding for Obama’s immigration move–something that might have altered the debate in Iowa’s Senate race.

The same might have applied in North Carolina, where Hagan spent the summer running against what she called the harsh legislative actions led by her opponent, House Speaker Thom Tillis. Udall, who backed executive action, might have leveraged the immigration debate against GOP Rep. Corey Gardner, broadening his agenda beyond what supporters regard as a one-note obsession with women’s reproductive issues.

“The Democrats got very little from delaying the executive action decision,” said Frank Sharry, executive director of America’s Voice. “They lost an opportunity to mobilize base voters, to send Republicans into crazy land and to consolidate the fast-growing Latino community as a core constituency in the Democratic coalition.”

Mr. Obama’s problems were on vivid and grating display Sunday in Bridgeport, Conn., when Latino activists interrupted his rep-election pitch for embattled Gov. Dannel Malloy six times. Latino protesters have heckled Obama frequently during his few campaign stops this cycle, routinely picket outside the White House and have also disrupted fund-raisers.

At one point Sunday, Obama told the hecklers “Shame on you.” Later, Obama told the hecklers they didn’t understand the basics.

“Hold on a second, everybody,” Obama said. “Quiet down. It’s a choice that we’ve got to make between two very different visions of America. I am sympathetic to those who are concerned about immigration. That’s why we fought for immigration reform. It’s the other party that’s blocked it. Unfortunately, folks get frustrated and so they want to yell at everybody.”

House Republicans refused to bring any immigration reform bills to the floor, killing the issue and prompting Obama to investigate and then promise to legalize millions without congressional consent. What’s now gnawing at the White House is the realization that failing to act may not save vulnerable southern Senate Democrats and may jeopardize other Democrats central to maintaining a Democratic majority.

As for Obama’s post-election plans, White House Press Secretary Josh Earnest refused to discuss any agenda items or deliberations on dealing with a possible GOP majority. He did make one promise, though.

“The one thing we know that’s going to happen before the end of the year is the president’s going to take action to use his executive authority to fix those aspects of our broken immigration system that can be fixed using executive authority,” Earnest said.

It’s a promise Latino activists have heard before. For Senate Democrats and the White House that cherishes its current majority, it may arrive too late.