For California Democrats, sprawling Los Angeles County is the scary-looking guard dog that just won’t bark.

In November’s election, California’s largest county was dead last in turnout, with just over 31 percent of registered voters casting ballots. And even that dismal number was a huge improvement from the June primary, when Los Angeles County turnout was 16.9 percent — also the lowest in the state.

The dismal local turnout makes a difference, particularly to Southern California politicians who aspire to statewide office, like former Los Angeles Mayor Antonio Villaraigosa and former Rep. Jane Harman. Both have been mentioned as potential candidates for the seat that Democratic Sen. Barbara Boxer will be giving up after 2016.

Plenty of politicians, consultants and academics blame the county’s turnout woes on the mail.

“Los Angeles County is still suffering the effects of not embracing vote-by-mail years ago,” said Douglas Herman, a Democratic political consultant in Pasadena.

That’s important, because studies have shown people who automatically get their ballots by mail are more likely to vote. In November, for example, 38 percent of Los Angeles County’s voters cast mail ballots. The next-lowest percentage of mail ballots was 57 percent in Humboldt County, which had a 52 percent turnout.

That low percentage of mailed votes in Los Angeles County is no accident. Years ago, county officials decided that encouraging residents to vote by mail would bury the election office under a flood of postal ballots. So while counties across California have spent the past 20 years making it easier for voters to use their mailbox as a polling place, the state’s most populous county refused to go along.

Those days are past, said Dean Logan, who has been the county’s elections chief since 2008.

“We now have the capacity to process more vote-by-mail ballots,” Logan said. “We’ve publicized VBM and encouraged people to sign up as permanent mail voters.”

Far below state’s average

But while the number of Los Angeles County voters who automatically get their ballots by mail has climbed from 14.3 percent in June 2010 to 29.6 percent in June 2014, that’s still well below the state’s 46.2 percent average.

In a county where long commutes can make it virtually impossible for some people to get to the polls, the obvious answer is vote-by-mail, said Eric Bauman, chairman of the Los Angeles County Democratic Party. “We have to get ballots into the hands of voters.”

With more than a quarter of California’s 38.3 million residents and almost a third of the state’s 7.7 million registered Democrats, Los Angeles County should be the political hub of the Golden State. Instead, Democrats whose political careers began in the Bay Area hold both U.S. Senate seats and five of California’s eight statewide offices.

“There are fewer people in the Bay Area, but more voters come out,” Herman said. “That has to be part of the calculation for any Los Angeles politician who wants to get in a statewide race.”

Herman got a sour taste of that turnout bias in June, when his candidate, former Assembly Speaker John Pérez of Los Angeles, finished third and out of the general election race for state controller, running behind Betty Yee, a Democratic Board of Equalization member from Alameda.

Ceding political decisions

It wouldn’t have taken many more votes from Pérez’s home turf to make up his 481-vote deficit to Yee, who beat Republican Ashley Swearengin in November.

“If Los Angeles voters decided to turn out, it could absolutely swing the outcome of California elections,” said Jessica Levinson, a clinical law professor at Loyola Law School in Los Angeles. “But now, we’re ceding many political decisions to Northern California.”

That’s one reason Lt. Gov. Gavin Newsom and Attorney General Kamala Harris, both former San Francisco officeholders, are the early leaders in the possible race for Boxer’s Senate seat. As statewide officeholders from Northern California, they have an advantage when it comes to moving up the political ladder.

Even without a big L.A. vote, statewide elections are still a steep climb for outnumbered Republicans. But it can be a different story locally. In November, a disproportionate turnout among older, whiter and wealthier — read Republican — voters cost Democrats some close legislative contests in Los Angeles County.

The low L.A. turnout also can be bad news for initiatives backed by Democrats.

“If a proposition isn’t getting those good Democratic votes from Los Angeles, there’s way less likelihood of success,” Herman said.

Jamie Court, president of Consumer Watchdog in Santa Monica, saw two initiatives his group sponsored go down in November. While both measures, Propositions 45 and 46, were heavily outspent, the low turnout doomed them, he said.

“There used to be what was called 'the L.A. County dump,’ which was the surge of Democratic votes that would show up late on election night,” Court said. In November, “it wasn’t there.”

Why they don’t vote

There are plenty of reasons county voters are avoiding the polls. Los Angeles is home to a young, transient and heavily ethnic population, all groups that seldom are regular voters. Add that to a general lack of concern about politics, and you have a recipe for disinterest on election day, Levinson said.

“The county is so spread out and voters have so little contact with their elected officials that Angelenos often don’t think about how state and local government can affect their lives,” she said.

Although there’s no single solution to the county’s turnout problem, boosting the mail vote could help. The county Democratic Party plans to push vote-by-mail in working-class communities and among young people, Bauman said.

Education needed

But county officials still have concerns.

“Vote-by-mail has to be looked at,” said Logan, the county registrar. “Some counties have pushed VBM and made efforts to present it as the only option, but there are certain areas of this county where that’s never going to work.”

Democrats agree that abysmal turnout in what should be the party’s bastion is a political disaster waiting to happen.

“There needs to be a major education effort,” said Herman, the Democratic consultant. Los Angeles County’s long habit of ignoring the trend toward mail voting, he said, “is a setback that will take years to resolve.”

John Wildermuth is a San Francisco Chronicle staff writer. E-mail: [email protected]