Rep. Jeff Denham, R-Calif. Bill Clark | CQ Roll Call | Getty Images

Republican Rep. Jeff Denham and his Democratic challenger, Josh Harder, are in one of California's hardest-fought congressional races, which has drawn national interest and increasingly become highly personal and bitter. The two are political polar opposites — Denham, an almond farmer and conservative ally of President Donald Trump, and Harder, a venture capitalist and moderate who supports Obamacare. The two also have clashed on issues such as taxes, immigration and water policy. Harder, a 32-year-old who recently worked at tech investment firm Bessemer Venture Partners, told CNBC in a recent interview he became involved in politics after Denham voted to repeal the Obama-era Affordable Care Act. He said the issue is very personal to him, since he has a younger brother who was born 10 weeks premature and has a pre-existing condition that could be lost without protections under so-called Obamacare.

Josh Harder (right), Democratic candidate for California's 10th Congressional District, visits the home of Alphabet employee Prem Ramaswami (left) in Mountain View. Source: Preston Merchant

"I decided I had to get on in and see what I could do to fix it," Harder said. Denham, 51, is seeking a fifth term to represent California's 10th Congressional District, which includes the northern portion of the San Joaquin Valley — a rural region known for its extensive agriculture production.

Prior to serving in Congress, Denham was a member of the California state senate and served in the Air Force for 16 years, on both active and reserve duty. Denham declined an interview for this story. The Republican congressman has labeled Harder as being part of the "core of the liberal elite," and the incumbent's campaign has sought to demonize Silicon Valley and the Bay Area. During a debate Saturday, the Republican congressman accused the venture capitalist of raising health-care rates after buying a company and in another instance "shipping jobs to India." "You want to attack me for my business record, and I think you know that's a bucket of lies," Harder said during Saturday's debate. "You're just trying to make me seem as scary as possible because you know that you can't defend the actual records that you voted for in Congress." Water is a hot issue in the district, where top employers include wineries and food companies. There has also been the loss of hundreds of jobs from the closure this year of local plants, such as Seneca Foods fruit processing and packaging plant in Modesto. Denham has sponsored legislation to bring more water to the Central Valley and increase water storage. However, Harder charged Saturday that his opponent during eight years serving in Congress "has not gotten a single dollar — not one federal dollar — of money for water infrastructure" for the 10th District. "We call him 'Bay Area Harder' because he aligns himself with the Bay Area and their issues," Denham responded in the debate. "I'm not for the Bay Area and their water grab to push more of our water out to the ocean." Some San Joaquin Valley farmers have been critical of the Bay Area for diverting water that they believe should go to fill the needs of the Central Valley, including to help grow various crops. Two nonpartisan newsletters that track House races — Cook Political Report and Sabato's Crystal Ball, from the University of Virginia — rate the Denham-Harder race as a "toss-up." Yet the forecasting and data site FiveThirtyEight estimates there is a "5 in 8 chance" that the Democrat will win. Denham won the last general election against a Democrat by just 3 points, whereas before it had been a much wider margin of victory. And Democrat Hillary Clinton carried the district in the 2016 election. "It's a close race," said Stephen Routh, a political science professor at California State University-Stanislaus. "He's running a good, effective campaign against a well-entrenched incumbent." The Democrats are hoping to pick up several House seats in California and other close races around the nation to wrest back control of the lower chamber they lost to the GOP in the 2006 midterm elections. Earlier this month, former President endorsed seven Democratic congressional candidates running in California, including Harder. The 10th Congressional District in Stanislaus County and portions of San Joaquin County is home to a growing number of residents who work in the San Francisco Bay Area but live further inland in cities such as Modesto and Turlock, where housing costs tend to be substantially lower. "You definitely see this trending in favor of Democrats in this area," said Routh, the local political scientist. "A big part of that which you can't discount is that the Bay Area living costs are ridiculous, so you've got more and more commuters coming out to the Central Valley, where housing is much more affordable. And these people who have jobs in the Bay Area generally are more of a liberal or blue mindset."

According to the latest state data, there are slightly more Democrats registered in the district than Republicans. Unemployment rates in the district have historically run higher than the state average. "Denham is a good quality guy, and he's done a good job representing this area," said Stanislaus County Supervisor Jim DeMartini, a farmer and chairman of the county's Republican Party. "He's running against a guy who gets all his money from the Bay Area. He can't raise anything locally." Harder countered that "the vast majority" of the campaign's contributions have been small dollar amounts raised locally that don't show up on federal election data. He also said the campaign is steering away from PAC money but charged that 70 percent of the money raised by his challenger has been from "corporate PACs and special interests, which is who he is working for. That's why he voted the way he did in favor of the tax bill, on the water bill."