Democrat Josh Harder pulled ahead of GOP Rep. Jeff Denham on Friday evening in the closely watched contest for a Central Valley seat.

The 1,300-vote lead that Denham, R-Turlock (Stanislaus County), had held disappeared when the results of late-arriving ballots were released. Harder, a former venture capitalist making his first run for office, now has about a 3,200-vote advantage, with an unknown number of votes still to be tallied.

The Central Valley contest is one of four California congressional races that will help determine the strength of Democrats’ control of the House for the next two years, but were too close to call Friday, three days after election day.

With piles of late-arriving mail and provisional ballots left to count, the campaigns were being cagey about making any predictions.

“No statement yet,” said Luis Vizcaino, a top consultant for Democrat Katie Porter, who was running about 4,000 votes behind Republican Rep. Mimi Walters of Irvine (Orange County). “Lots of votes still to be counted.”

That’s a tale that can be told across California. On Friday morning, the secretary of state’s office reported that there were more than 4.8 million votes left to count in the state.

In Orange County alone, Registrar Neal Kelly said as many as 450,000 ballots were left to tally after election day, leaving some of the state’s hottest congressional races in a vote-counting limbo.

“We could be counting past Thanksgiving,” he said.

That makes for nervous times in some of the state’s most closely contested congressional contests. Democrats targeted seven GOP seats statewide because Hillary Clinton won those districts in the 2016 presidential election, but so far the results are clear in only three. Two have gone to Democrats and one to the Republican incumbent.

Three of the four seats that are still up in the air are in Southern California.

In the 39th District, Republican Young Kim of Fullerton (Orange County), a former assemblywoman, has seen her lead shrink since election day as more of those late votes have been counted. The five-point margin she held over Democrat Gil Cisneros is now just over two points, 51.1 to 48.9 percent, a lead of about 3,400 votes. The seat is now filled by retiring GOP Rep. Ed Royce.

Democrats hold a growing lead in the 48th District, where businessman Harley Rouda is challenging 30-year GOP Rep. Dana Rohrabacher of Costa Mesa (Orange County). Rouda is ahead by about 4,700 votes, 51 to 49 percent, a lead has nearly doubled since election day.

Walters holds a 51 to 49 percent edge over Porter, a UC Irvine law professor, but that difference has shrunk as well since election day.

There are an estimated 70,000 votes or more to count in those Orange County districts.

California’s tightest congressional race remains the one in the Central Valley.

“This race is still too close to call. That means we still have a fight ahead of us and we still need your help,” Harder said in a post-election fundraising email to supporters. “We’ll need to keep our staff on for a little longer, hire a lawyer and make sure we have everything we need to win this fair and square.”

The prospect of a long count in tight races has tempers fraying.

In an email Friday, the Cisneros campaign charged that the Democratic candidate’s home in Yorba Linda (Orange County) home was vandalized and that someone shot out the window of a campaign office on election night.

The huge number of uncounted ballots also leaves a couple of statewide races in play.

In the contest for state insurance commissioner, Democrat Ricardo Lara leads independent Steve Poizner by about 120,000 votes.

In the nonpartisan contest for state superintendent of public instruction, educational businessman Marshall Tuck holds a 69,000-vote lead over Assemblyman Tony Thurmond, D-Richmond.

John Wildermuth is a San Francisco Chronicle staff writer. Email: jwildermuth@sfchronicle.com Twitter: @jfwildermuth