Rep. Jeff Denham, R-Calif., won his primary Tuesday — but with 37.7 percent.
He did finish well ahead of Josh Harder, a Democrat who took about 16 percent of the vote and ran second, narrowly scraping past Republican Ted Howze.
But Denham's total was 10 points lower than his 2016 primary vote, and 20 points less than his 2014 primary showing.
Denham, considered a vulnerable Republican since Democratic presidential candidate Hillary Clinton won his district in 2016, generally has to perform a balancing act between conservatives and centrists.
Democrats say that's gotten a lot harder in the age of President Donald Trump, but Republicans publicly shrugged off the lower numbers, dismissing Harder as an easy candidate for Denham to defeat in November.
"Historically, Denham has always been a top target, but he's squeaked by," said John Vigna, communications director for the California Democratic Party. "This year is genuinely different, because for Democrats it's not just about defeating Denham, it's about taking back the House."
Drew Godinich, spokesman for the Democratic Congressional Campaign Committee, called Tuesday's result Denham's "weakest performance yet."
Republicans countered that all the Democratic candidates combined still hadn't hit a majority in the district.
The combined Republican vote tops 50 there, and Dems just nominated a Bay Area investment banker who is far too liberal for the district," said Jack Pandol, a spokesman for the National Republican Congressional Committee. "We have every reason to be confident in the 10th."
Harder grew up in Turlock but is painted as an out-of-touch resident of San Francisco by his opponents due to his former career as a venture capitalist. He now teaches business at Modesto Junior College.
Denham said Wednesday that between him and Howze, he was encouraged by Republican turnout in the district. But Democrats said Howze's support is a sign that Denham's careful "threading the needle" to appeal to both centrists and a conservative base is going to get harder for him.
Howze attacked Denham during the campaign for pushing a vote on immigration that would provide a special pathway to citizenship for certain immigrants known as Dreamers.
"This isn't going to help him with Democrats, because it isn't just about him — they want to take back the House," Vigna said, referring to the immigration push. "But it does alienate his base."
Joshua Whitfield, Denham's campaign manager, said while Howze and others employed a "scare tactics campaign" based on the push for immigration action, he believed there would be more opportunities to educate conservative voters before November, and they would come around.
The vast majority of voters in the district "understand this is a broken immigration system that needs to be fixed, no matter what ultra-conservative websites say," he said.
"The bottom line is, there is not one sign of the blue wave in the 10th Congressional District," Whitfield said.