Republican Rep. Jeff Denham walks the line in San Joaquin Valley | McClatchy Washington Bureau

Rep. Jeff Denham, R-Turlock, can really lower the boom.

His voice is emphatic and unmistakable, especially when he gets rolling; when he’s lamenting California water shortages, say, or blasting high-speed rail or rallying behind his fellow veterans.

“He’s not shy about telling people where he’s coming from,” said Rep. Jim Costa, D-Fresno.

Politically, too, the 48-year-old Denham can stand out, though he doesn’t always choose to. His can be a complicated portrait as he prepares to face voters again in the June 7 primary.

Denham voted with a majority of the Republican members of the House of Representatives 91 percent of the time in 2014 and 87 percent of the time last year, tallies by CQ Weekly show. He’s a member of the extended House whip team, a vote-counting job that rewards the dutiful.

But sometimes, Denham can break ranks. His party unity score last year was lower than the House GOP average of 92 percent, and it was the lowest among California’s House Republicans. He’s not adverse to pairing with Democrats.

“Jeff and I have worked together on things going way back to our time together in the state Legislature,” said Rep. Jared Huffman, D-San Rafael. “We disagree mightily on California water policies, but we’re always trying to find common ground where we can.”

Huffman and Denham, for instance, co-sponsored a bill putting about 511 acres in Sonoma County into trust for the Lytton Band of Pomo Indians. With Rep. Sam Farr, D-Carmel, Denham has put together legislation that reopens Clear Creek Management Area lands in Fresno and San Benito counties to off-road vehicles, in exchange for designating other lands as wilderness or as wild-and-scenic river status.

On the wall of his seventh-floor House office is a President Barack Obama-signed copy of a veterans jobs bill he co-authored with Rep. Tim Walz, a member of the Minnesota Democratic-Farmer-Labor Party. With Rep. Jerry McNerney, D-Stockton, he successfully pushed for construction of a $280 million veterans medical facility in French Camp.

Now seeking his fourth term, following eight years in the state Senate, the self-described conservative Republican recognizes the fact that registered Republicans have only a 2-percentage-point advantage over registered Democrats in his district spanning Stanislaus and San Joaquin counties.

Two Democrats and one Republican are challenging him in the primary.

“Whether it was in the state Senate or here in Congress, I don’t necessarily follow party lines,” Denham said in an interview. “I follow the issues I feel are important to my personal beliefs, to my community and to my district.”

Still, party lines can impose certain limits.

Last Thursday, while House Democrats shouted “shame,” Denham was one of seven Republicans to switch their votes from yes to no on a Democrat’s defense bill amendment described as protecting gay and lesbian rights. The switches came after normal voting time had expired, and followed apparent arm-twisting by GOP leaders.

Denham did not respond to a request for comment on the vote.

“They will have themselves to look at tonight in the mirror and explain to themselves whether their first vote was a principled vote or whether they had a Damascus Road experience in the few minutes that transpired” between votes, House Minority Whip Steny Hoyer. D-Md., said on the House floor.

In 2013, Denham became the first House Republican to endorse a comprehensive immigration-revision package that included a path toward citizenship for immigrants currently in the United States illegally. He calls it “a personal issue for me,” citing his Mexican-born father-in-law’s quest for U.S. citizenship as well as his own experiences as an Air Force enlisted man.

“I was serving with many immigrants who were gaining their citizenship while serving with me,” Denham said adding, “The person I reported to on active duty was Filipino, getting his citizenship.”

Constituent demographics, too, command his attention. More than 42 percent of the residents of his 10th Congressional District are recorded as Hispanic or Latino. More than 154,000 were born in foreign countries, according to the census. This combination of the personal and the political was in the background when Denham voiced support for overhauling immigration law, and when he offered a bill he called the ENLIST Act.

The bill would provide legal permanent-resident status for immigrants in the U.S. illegally who serve in the U.S. military. In 2013, Denham offered it as an amendment to a defense bill. Then, at the request of then-House Majority Leader Eric Cantor, R-Va., Denham withdrew it with the understanding that it would be considered as part of a broader immigration bill.

A tea party conservative subsequently defeated Cantor in a primary, chilling further Republican consideration of immigration restructuring, though Denham says he won’t relent.

“I’m going to continue to bring that up,” Denham said of his measure, later acknowledging that in Congress, in general, “things take too long, and they’re certainly too political.”