Rep. Jeff Denham of Turlock signed on over the weekend to a comprehensive immigration measure that has gone nowhere in the Republican-led House. (The measure is a tinkered-with version of a bill that passed in the Democratically-controlled Senate.)

Denham thus became the first Republican to publicly support the comprehensive House measure, though support for immigration reform is not a new thing for him. Some of it is personal: His wife is the daughter of a legal immigrant from Mexico. His sister married a man who entered the country illegally but has since gained legal status.

But there is an undeniable political imperative pressuring Republicans like Denham and another GOP member from further south, Rep. David Valadao of Hanford, who also has expressed support for immigration measures so far blocked in the House.

The numbers are stark, and moving against Republicans.

Denham’s Modesto-area district is, according to the California Target Book, a nonpartisan political handbook, 40% Latino by residency. So far, only 25% of registered voters are Latino but that number is growing. The district is split between Democrats and Republicans, with about 4 in 10 voters from each, and nonpartisan voters — who usually side with Democrats statewide — making up the difference.

In Valadao’s district, according to Target Book calculations, 71% of the residents are Latino, and Latinos make up more than half the registered voters. The first-term congressman is being challenged next year by Amanda Renteria, a Tulare native who until recently worked in Washington for Democratic Michigan Sen. Debbie Stabenow and who has already garnered moneyed endorsements. That district is even more Democratic than Denham’s, just shy of half.

Several dynamics are in play here. First and most obvious is the growing percentage of Latino residents and voters, who to this point in California and nationally have been highly loyal to Democrats. The number of Asian and nonpartisan voters, who also take cues about candidates from their immigration positions, are also growing.

Also contributing to the threat to Republicans in California is the state’s changing political geography. As urban populations and housing prices swell, voters have been pushed out to the suburbs and exurbs, where their views moderate the previous conservative nature of the places they inhabit. Such is the case in Denham’s district, and in districts like the one in San Bernardino County represented by incumbent Gary G. Miller, who is high on the get list for Democrats in 2014.

Just about half of the residents in Miller’s district are Latino, as are 33% of its voters. Democrats outnumber Republicans, 41% to 34%.

Republicans nationally are counting on a predictable drop in Democratic turnout from the presidential contest last year to ease their incumbents into another term. But for the candidates in competitive districts, there is also a need to stand out from the unpopular GOP brand. And that is another thing Denham accomplished with his high-profile embrace of the comprehensive measure.

Timing is everything: Denham’s announcement came just after Latino fundraisers announced via a Washington Post story that they planned to spend up to $20 million targeting members of Congress who oppose the broad immigration measure at the same time they represent large Latino populations.

On a conference call Monday with immigration reform advocates, Denham was repeatedly praised for what participants called a bold and courageous gesture. Denham, who is still in the process of trying to convince others in his party to join him, alluded to the danger that bottling up immigration reform could have on his party.

“Certainly it’s important for my community; it affects every member of my community,” he said. “I would make the case to Republicans not in California or border states that this is an important American issue.”

Noting that some Republicans “have said some pretty outrageous things” about immigration, he asserted that it was important for the party to have a positive message to offer.

But it remains easier to see Republicans in Congress wanting to table the issue, with an election year looming and no hard and fast date by which it has to be passed.

“The biggest challenge with immigration reform is it doesn’t have a deadline,” he said.

cathleen.decker@latimes.com

Twitter: @cathleendecker