Early-voting Democrats could be the key in some of the state’s most fiercely contested congressional races, new vote-by-mail statistics show.

More than 3.7 million Californians already have voted in Tuesday’s election, with hundreds of thousands more mail ballots expected to arrive at county election offices over the next few days. And in the handful of toss-up congressional contests across the state, more and more of those ballots are coming from Democrats.

“All around the state, we’re seeing an underperformance by Republicans (in early voting) compared to 2012,” said Paul Mitchell, vice president of Political Data Inc., which provides information on voters and voting to both Republican and Democratic campaigns. “Vote-by-mail means higher turnout ... and it looks like Democrats are getting out more of (those voters) than in the past.”

While the raw number of early voters hasn’t changed much from the 2012 election, when President Obama won an overwhelming re-election victory in California, the composition of that vote is different. In district after district, Democrats are outpacing their early voting numbers from four years ago, either expanding the lead they had or cutting into GOP margins.

In rural Sacramento County’s Seventh Congressional District, Democrat Rep. Ami Bera is facing a tough challenge from Republican Scott Jones, the county sheriff.

In 2014, when Bera eked out a 50.4 percent to 49.6 percent win over the GOP’s Doug Ose, 41 percent of the mailed ballots were from Democrats, with 40 percent from Republicans. But as of Nov. 2, 44 percent of the early voters were Democrats, with Republicans making up 36 percent, according to figures posted by Mitchell.

That same effect is being seen in strong Republican areas like the 49th Congressional District, which straddles the Orange County and San Diego County border.

Despite a 39 percent to 31 percent Republican registration edge, veteran GOP Rep. Darrell Issa is a top Democratic target. But the Republicans’ 16-percentage-point lead in early voting four years ago has slipped to eight points this year, which could be bad news for Issa in a close contest with Democrat Doug Applegate.

That Democratic surge is strategy, not coincidence, said Michael Soller, a spokesman for the California Democratic Party. A few years ago, the party tested an effort dubbed “Gamechanger” in Southern California. It involved a concerted effort to persuade Democrats who voted only occasionally, if at all, to sign up for vote-by-mail and cast their ballots before election day.

“We found that these people who signed up for vote-by-mail voted at a 12 percent higher rate than Democrats as a whole,” Soller said. “The project proved that if you can sign them up, you can turn them out” to vote.

Democrats have been running similar efforts across the state this year. Since early October, the state party has had paid staff members in many of California’s most contested congressional and legislative districts, working to pump up that early vote, he added.

But Republicans question just how effective those Democratic early vote efforts will be.

In the 25th Congressional District, which stretches from Palmdale and Lancaster in the desert reaches of Los Angeles County west to the city of Simi Valley in Ventura County, GOP Rep. Steve Knight is going all out to win a second term in Congress.

The raw numbers look good for Democrat Bryan Caforio. The district’s registration has shifted from a Republican lead of 39 percent to 36 percent in 2012 to a current 38 percent to 35 percent Democratic edge. While Republicans now hold a five -percentage-point margin among early voters, that’s down dramatically from 17 points in 2012.

Yet Matt Rexroad, a political consultant working for Knight, is pleased with the direction of the race, arguing that the former state legislator fits the district and that “we’ve never had a survey that has shown Knight behind.”

“Democrats typically register a lot of people, but not that many turn out to vote,” he said. “They register a transient population, people who don’t stay around long.”

Harmeet Dhillon, former head of the San Francisco Republican Party and the Republicans’ current national committeewoman from California, argued that mail ballots, which have been arriving since early October, don’t always reflect the changing reality of a political race.

The new disclosure of a renewed FBI email investigation “could be dampening enthusiasm for (Democratic presidential candidate) Hillary Clinton,” she said.

Early voters, who include those who turn in their mail ballots at the polls on election day, now represent a majority of the ballots cast. In the 2012 general election, 51 percent of the ballots were vote-by-mail.

The Democrat-leaning voting numbers may simply be mirroring a new reality, said Mitchell, who updates the vote-by-mail numbers daily.

“Nearly 5 million people have either registered to vote or updated their registration since January, and 72 percent of them signed up as permanent vote-by-mail,” he said. That could mean the changing figures merely represent a front-loading of the young, minority and Democratic voters who dominate the rolls of new voters, and not an overall change.

Then there’s the question of just who these early voters are. While about half of voters 65 and older have returned their mail ballots, that number falls to 10 percent for those 25 and younger, Mitchell said.

“That’s an amazing number,” he added. “Campaigns say a lot of young people are sitting on their ballots.”

That generation gap makes a real difference. A Field Poll released Thursday found that Republican Donald Trump led Clinton, 52 percent to 40 percent, among California voters 65 and older. Among voters 18 to 39, it was Clinton 68, Trump 17.

“More voters are coming into the process, and more of them are Millennials and more of them are Democrats,” Mitchell said. “Now we have to see whether they are enthusiastic enough to vote.”

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