Democrats are on the verge of sweeping the seven Republican-held House seats they put at the top of their target list in the midterm elections in California, as Democratic businessman T.J. Cox has taken the lead over GOP Rep. David Valadao in a San Joaquin Valley contest.

Cox was leading by 436 votes Monday after the latest ballots from Kern County were counted — 55,672 to Valadao’s 55,236, or 50.2 to 49.8 percent.

“Here in the Central Valley, we are often underestimated and counted out,” Cox said in a statement. “But while the national spotlight focused elsewhere, our hard-working communities came together to fight for better health care, good jobs, and more opportunities.”

Democrats have already won six House districts in California that were held by Republicans, but where Hillary Clinton out-polled Donald Trump in the 2016 presidential election. Valadao, R-Hanford (Kings County), was the incumbent in the seventh.

The list of flipped GOP seats list includes four representing Orange County, the Republican Party’s longtime California stronghold.

If Cox’s lead holds, it would give Democrats a 40-seat gain in the House nationally. The party is already assured of control of the chamber next year.

Valadao was seen as resistant to the Democratic wave, even though he voted with President Trump nearly 99 percent of the time, according to fivethirtyeight.com. Although Democrats hold a 16-point voter registration advantage over Republicans in Valadao’s 21st Congressional District, he has been comfortably re-elected twice. But like other Republican candidates around the state, his position steadily worsened as late-arriving ballots broke strongly in Democrats’ favor.

Cox originally intended to run for the 10th District seat held by Republican Rep. Jeff Denham of Turlock (Stanislaus County). But the Democratic Congressional Campaign Committee was concerned that too many Democrats were trying to take on Denham and not enough viable candidates challenging Valadao, so it persuaded Cox to run in the 21st District. Cox does not live in the district.

Denham was defeated in the Nov. 6 election by Democratic venture capitalist Josh Harder.

Joe Garofoli is The San Francisco Chronicle’s senior political writer. Email: jgarofoli@sfchronicle.com Twitter: @joegarofoli