The state’s primary election season reaches its traditional climax Tuesday when voters troop to the polls to decide which candidates they will choose from in November.

But in California, this election has been anything but traditional.

Across the state, voters have been casting mail ballots for almost a month. Rather than ending when the polls close at 8 p.m. Tuesday, the election will continue late into the week, with ballots postmarked by election day added to the official mix. And as California election officials have found in years past, the growing number of late-arriving mail ballots means it could be a week or more before all the votes are counted and the winners reported.

While the polls don’t open until 7 a.m. Tuesday, it already looks as if the voter turnout could be disappointing, especially for Democrats who are hoping the state’s raucous opposition movement to President Trump will take its protest to the ballot box.

“There’s been a lot of conversation about a heightened sense of political engagement bringing out more voters, but there’s no evidence of it” in the number of ballots already mailed in, said Paul Mitchell, vice president of Political Data, which provides information about turnout and voter registration to political groups.

“There might be more turnout at the polls,” Mitchell said, “but I wouldn’t hold my breath.”

It’s not just the statewide numbers that seem low. In San Francisco, early mail and in-person voting is falling below expectations, said John Arntz, the city’s elections director. The city has received about 74,000 early ballots, for a pre-election day turnout of 15 percent.

“We received 6,000 vote-by-mail ballots (Monday), and we certainly expected more the day before the election,” Arntz said. “The numbers seem to point to a lower turnout than we expected.”

That’s surprising in San Francisco, where the ballot includes a tight special election for mayor and an expensive battle over Proposition E, a city ban on flavored tobacco products. San Franciscans will also join voters in eight other Bay Area counties in deciding the fate of Regional Measure 3, which would raise tolls on state-owned bridges to pay for transportation improvements.

In San Mateo County, voters were dealing with an election where everyone received a ballot in the mail and more than 200 local polling places were replaced by 39 voting centers. That system will be in place around the state in 2020.

“We’ve received about 96 percent of our votes by mail, which shows our message has been received,” said Mark Church, the county clerk and elections chief. “As of the weekend, we’ve received about 76,000 mail ballots, for a turnout of 20 percent so far.”

Church predicted a final county turnout of 35 percent, above the 27 percent total in the 2014 primary.

The mail ballot totals are increasingly important, since fewer and fewer people step inside a voting booth. In 2014, about 69 percent of the primary election vote was cast by mail.

A growing number of young people receive their ballots in the mail — but that doesn’t mean they’re sending them back in, said Mitchell, whose company tracks those data.

“It’s strange that young and old voters together account for about 50 percent of the state’s mail ballot,” he said. “Yet in one group, half of them have returned their ballot, while in the other it’s 9 percent.”

With voters 65 and up providing such a large percentage of the early mail responses, those primary ballots are likely to come from voters richer, whiter and more conservative than California’s electorate as a whole, Mitchell said.

That’s bad news for Democrats, since a strong showing by voters 18 to 32 years of age is key to their efforts to flip seven or more GOP-held congressional seats in California. That effort, in turn, is crucial to the national party’s hopes of winning back the House in November.

The average statewide turnout for the past four midterm primaries has been about 34 percent, although that number skidded to 25.1 percent in 2014.

“Think of those numbers as goalposts, with the actual turnout falling somewhere in between,” Mitchell said.

For the candidates, the location of those voters is just as important as their numbers. While the Democratic contenders for governor — Lt. Gov. Gavin Newsom, former Los Angeles Mayor Antonio Villaraigosa, state Treasurer John Chiang and Delaine Eastin, the former state schools chief — have been barnstorming across the state in search of votes, they’re putting their money and TV ads where their votes are.

In recent days, for example, Villaraigosa has been virtually invisible on the Bay Area airwaves as he focuses his TV ads on parts of the state where he hopes to pick up more support, such as Southern California and the Central Valley.

The main Republicans in the race, San Diego-area businessman John Cox and Orange County Assemblyman Travis Allen, spent the campaign’s last days on their home ground.

As usual, Los Angeles County will be the state’s biggest question mark on election night.

The county has about 27 percent of all of California’s registered voters. It also has one of the state’s lowest percentages of early voters, which means that a huge proportion of the L.A. vote is cast at the polls and is reported late on election night.

Statewide, however, the popularity of mail ballots means that 30 to 40 percent of all the votes cast will be reported between 8 and 8:30 p.m. on election night, providing more than a hint of the final results.

The state’s primary system calls for the two leading vote-getters in all federal and state legislative races, regardless of party, to advance to the November election. The lone exception is the nonpartisan race for state superintendent of public instruction, where if one candidate receives more than 50 percent of the vote in the primary, he or she is automatically elected without a November runoff.

Besides the San Francisco contests and Regional Measure 3, hotly contested district attorney races in Alameda and Contra Costa counties, along with Napa’s Measure C, which would limit vineyard development to protect the county’s natural habitat, have attracted wide attention. So has a Santa Clara County vote on whether to recall Superior Court Judge Aaron Persky for his sentencing of a Stanford University athlete to six months in prison for a sexual assault conviction.

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