Stung by recent election defeats, Democrats are leading the charge to lower the voting age to 17, with a little help from liberal billionaire George Soros.

In California, Democratic legislators introduced this week a landmark bill, ACA 10, that would give the Golden State the nation’s youngest statewide voting age by lowering the threshold from 18 to 17 in the name of reversing the slide in voter turnout.

“Young people are our future,” said Democratic Assembly member Evan Low, the measure’s sponsor. “Lowering the voting age will help give them a voice in the democratic process and instill a lifelong habit of voting.”

The proposal comes as the most ambitious of a host of efforts to chip away at the 18-year-old voting age as Democrats seek to bring into the fold younger voters, who traditionally support more liberal causes and candidates than do their elders.

Mr. Soros is on board: His Open Society Foundations is among the left-wing philanthropies backing FairVote, which has pushed to allow 17-year-olds who will turn 18 before the general election to vote in presidential primaries and caucuses, a policy now on the books in 21 states and the District of Columbia.

Also gaining popularity is preregistration. Twenty states and the District of Columbia allow certain minors, ranging in age from 16 to 17 years and 10 months, to register to vote before turning 18, according to the National Conference of State Legislatures.

California Secretary of State Alex Padilla appeared Thursday at John F. Kennedy High School in Los Angeles to announce the state’s online preregistration system for 16- and 17-year-olds, joined by the program director of YVote.

The group, which had advocated for preregistration, also has a Soros connection. YVote is a project of the Movement Strategy Center, which receives donations through the Funders for Justice, whose work is funded by left-wing philanthropies including Open Society.

In November, voters in Berkeley, California, took it a step further by lowering the voting age for school board elections to 16. Two Maryland cities — Takoma Park and Hyattsville — have in recent years allowed 16-year-olds to participate in municipal elections.

Proponents argue that the 18-year-old threshold is unfair and arbitrary, but there is little doubt that lowering the voting age disproportionately benefits one side of the aisle, and it’s not the right.

In November, Republican Donald Trump received about 37 percent of the 18-29 vote, about the same as did Mitt Romney in 2012, while Democrat Hillary Clinton won about 55 percent, according to the Center for Information & Research on Civic Learning and Engagement at Tufts University.

The California numbers were even more skewed: Mr. Trump received just 18 percent support from Golden State voters ages 18-24, “and 17-year-olds would be just as heavily Democratic,” said John J. Pitney Jr., American politics professor at Claremont McKenna College in California.

“It is a transparent ploy to pad the Democratic vote,” said Mr. Pitney.

California lawmakers have been wrestling with ways to increase voter engagement since 2014, when just 42.4 percent voted in the nonpresidential election year, the lowest percentage in modern state history.

That year, Mr. Low noted that only 8.2 percent of eligible California 18- to 24-year-olds voted. Yet adding more teenage voters would depress turnout percentages if 17-year-olds follow the same trend as their slightly older peers.

“This bill makes no sense,” said Mr. Pitney. “It is highly likely that participation of 17-year-olds would be even lower than for 18-year-olds, so the effect would be to pull down the overall turnout rate.”

Republicans have long argued that the reason for listless voter engagement is a lack of competition. Democrats rule California as a virtual one-party state, holding all statewide offices and both legislative chambers, while most of the state’s congressional districts are safe seats.

In fact, voter participation rose in November as 75.3 percent of registered California voters cast ballots, fueled by a tight, contentious presidential contest between Mr. Trump and Mrs. Clinton.

The 2016 figure was about 3 percentage points higher than in 2012, when President Obama won re-election, but not as high as 2008, when 79.4 percent of voters turned out to elect Mr. Obama to his first term, the Sacramento Bee reported.

Mr. Low, who chairs the Assembly Elections and Redistricting Committee, argued that one reason for the low 18-year-old turnout is that many are transitioning from high school to college or to the workforce.

“Lowering the voting age to 17 will catch youth at a time when they are still connected to their school, their home, and their community,” said his statement.

Expanding the voting age in California won’t be easy: As a proposed state constitutional amendment, the measure would need a two-thirds vote from both legislative chambers before going before the voters on the state ballot.

If passed, 17-year-old Californians would be able to vote “in all public elections, including federal elections for president and Congress,” said Low spokeswoman Marly Young.

“The 26th Amendment to the United States Constitution prohibits states from setting a voting age above 18, but does not prohibit states from setting a voting age under 18,” Ms. Young said in an email.

More easily done is giving 17-year-olds poised to turn 18 before November the right to vote in primary and caucus elections, which can be done in some states by changing party rules, as opposed to state law.

Twenty-one states and the District of Columbia have extended primary voting to 17-year-olds. In four states — Alaska, Hawaii, Washington and Wyoming — those just shy of 18 may participate in Democratic but not Republican primaries, which FairVote described as confusing.

“This patchwork policy creates confusion and can potentially disenfranchise eligible voters,” said FairVote. “Parties should act nationally to make this practice a norm.”

Elsewhere, 17 is just the beginning, as evidenced by the “Vote16” movement, which pushes for lowering the voting age for local races, particularly those involving public schools.

“Research shows that 16- and 17-year-olds are intellectually ready to vote,” says Vote16USA, a project of Generation Citizen. “For example, on average 16-year-olds possess the same level of civic knowledge as 21-year-olds.”

Jimmy Sengenberger, the 26-year-old president of the conservative Millennial Policy Center, took issue with the younger-voter trend. He noted that the U.S. voting age was lowered in 1971 from 21 to 18 because it was seen as out of step with the age of adulthood.

The argument made at the time was that 18-year-olds were being sent to fight in the Vietnam War even though they could not vote at home.

“Unless we lower the legal age of adulthood, with all its rights and privileges, the right to help determine the future of our country through the vote should not be extended to anyone who will not be 18 on or before Election Day,” he said.

Not all liberals are in favor of giving younger teens access to the ballot. In November, San Francisco voters defeated by 52 percent to 47 percent a ballot measure permitting 16-year-old voters in city elections.

The problem for some voters? As the ballot argument against Proposition F put it, younger voters were seen as more inclined to “support free-spending candidates and issues than older and more business-oriented citizens.”