As they come of age, the number of Millennials — that enormous generation born in the 1980s and ’90s — registering to vote is growing so fast that they’re on the verge of overtaking the Baby Boomer behemoth as a share of the California electorate.

And new evidence confirms that, so far at least, the GOP is losing them.

Fewer than 1 out of every 5 Millennial voters in California is registered as a Republican. Not that the Republicans’ loss is the Democrats’ gain. Recent voter data show that Millennials are the driving force behind the huge growth in Californians registering with no party affiliation at all.

They’re also more optimistic than other generations — the latest polls show them more approving than their elders of the job the Legislature is doing, and far more supportive of the direction in which California is headed.

Their emerging dominance means that young voters are well-positioned this year to influence several critical questions — including the generational changing of the guard as two Democrats compete to replace retiring Sen. Barbara Boxer, and the fate of 17 ballot measures covering issues as varied as the death penalty, gun control and marijuana.

Many people point to Proposition 64, the initiative to legalize recreational use of marijuana, as the state issue over which Millennials are most likely to wield their power.

That was the prediction of Rep. Eric Swalwell, D-Dublin, as he addressed a recent UC Berkeley conference focused on the generation.

“So I hope young people show up” to vote, he said. “Because once California legalizes marijuana, I think the rest of the country is going to follow.”

Millennials make up 29 percent of the state’s registered voters, the second-largest share among five living generations. Only Baby Boomers, at 32 percent, make up a bigger bloc.

“You’ve got to think of Millennials as essentially becoming the largest single segment of the electorate,” said Paul Mitchell, vice president of Political Data Inc., who analyzed California voter registration data by generation.

Their dominance among California registered voters has been climbing steadily since the generation hit voting age in 2000. This spring, tech-savvy Millennials propelled a rise in voter registration: Nearly 200,000 Californians signed up to vote over two days in May when a Facebook campaign linked users to the state’s online registration portal. Nearly two-thirds of them were 35 or younger, according to Secretary of State Alex Padilla.

Now other tech companies are expressing interest in promoting voter drives, potentially further boosting registration among Millennials before the November election. Padilla said he is talking with Google and Twitter.

“It’s great to see the social media world really committing to this. It’s really helpful and powerful,” he said.

How potent this generation really is at the polls comes down to one question: Will they actually vote?

Although turnout among young people is typically low, they could be motivated this year by the turbulent presidential election, and their numbers would have a spillover effect on California races with long-lasting implications.

“If they vote, I think they will be significant in any number, if not all, of the initiatives,” said Gale Kaufman, a political consultant running campaigns for Prop. 64, to legalize marijuana, and Proposition 55 to extend temporary income tax increases for the wealthy.

“Issues like guns, the death penalty and marijuana should evoke a lot of interest at the campus level and moving through Millennials. Social issues or values issues tend to energize younger voters.”

Opponents of marijuana legalization plan to use a campaign designed to raise concerns about specific aspects of Prop. 64, especially those that could be of concern to parents.

“I think the concept polls well — I don’t think the details poll well,” said Andrew Acosta, a political consultant working on the campaign against Prop. 64. “Once you start to open this thing a little bit, there might be things that turn people off.”

But many Millennials have their minds made up. Voters ages 18 to 34 favor legalizing marijuana by 2 to 1, a far greater ratio than older voters, according to a Public Policy Institute of California poll in May.

Hours before Swalwell endorsed Prop. 64 in Berkeley, one conference participant said it was the only initiative on California’s long ballot this year that he was aware of — and he plans to vote yes.

“We’ve had medicinal marijuana in California for about 20 years,” said Chris Johnigan, 29, a substitute teacher who lives in the Bay Area. “It seems like we’re past due to have it legalized on a recreational level.”

Marijuana legalization may be the first issue in which Millennial voters cast the decisive votes in California. But it certainly won’t be the last.

CALmatters is a non-profit journalism venture dedicated to exploring state policy and politics. For more stories by Laurel Rosenhall, go to calmatters.org/newsanalysis