Donald Trump is only part of the challenge facing Democrats this fall — just as important is the generational split between supporters of Hillary Clinton and Bernie Sanders, and what that rift could mean for the future of the party.

“I have not seen anything like it since I’ve been polling,” said Mark DiCamillo, director of the Field Poll, which has been tracking California politics for decades.

Field’s final pre-election poll, taken at the end of May, showed voters ages 18 to 29 favoring Sanders over Clinton, 75 to 15 percent. The split was almost as lopsided among respondents in their 30s, who backed the Vermont senator by 56 to 33 percent.

Clinton’s base was above 40 and was most solid among the oldest voters: Those older than 65 favored the former secretary of state by a 2-to-1 ratio.

Race, gender, geography, it didn’t matter: The younger the voters, the stronger the tilt to Sanders.

“It’s something that we have seen in just about every primary race across the country,” said Mark Baldassare, who runs the Public Policy Institute of California.

Sanders’ candidacy has put a whole new set of issues on the table for the Democratic Party, largely centered around one theme — fear of the future.

“These are young people who are facing a world of economic uncertainty, and his call for things like free college has got them on the bandwagon,” said Henry Brady of UC Berkeley’s Goldman School of Public Policy.

“The question is, can Hillary Clinton come up with a message to bring them into the fold?” Brady said. “So far she hasn’t been able to do that.”

She tried again Tuesday. At her victory speech in Brooklyn, N.Y., Clinton said, “Sen. Sanders, his campaign and the vigorous debate that we’ve had about how to raise income, reduce inequality, increase upward mobility have been very good for the Democratic Party and for America.”

The reason the generation gap matters over the long term is that the young people flocking to Sanders will gradually become the Democrats’ mainstream voters. Like the Tea Party forces who burst on the scene in 2010, paving the way for Trump six years later, or the young Democrats who idolized Eugene McCarthy in 1968, Sanders voters are likely to be a power to reckon with for years to come.

“You are already seeing it with people like (Lt. Gov.) Gavin Newsom who are pivoting with issues like same-sex marriage and marijuana,” said Republican Bill Whalen, a research fellow with the Hoover Institution.

“I don’t see it so much as a clash as an awakening,” said Democratic operative Christine Pelosi, a Clinton supporter who also runs boot camps for potential officeholders.

“It’s the progressive future of the party,” she said. “We are going to have to make room for them.”

Money track: Hope you enjoyed seeing Hillary Clinton and Donald Trump on the California campaign trail. Chances are they won’t be staging repeat performances between now and November.

“We’re going back to our traditional role as the biggest ATM in America,” said Democratic fundraiser Wade Randlett. “If Donald Trump spends one minute in California not raising money, I will fall over dead.”

At the same time, look for regular cash runs by Clinton — about one every three to five weeks.

According to political fundraising site OpenSecrets.org, almost a quarter of the big-check money raised by Clinton’s campaign and super PAC during the primary season — $56.2 million — came from California.

Although Trump largely self-funded his primary run, Californians contributed 15 percent — or $651,798 — of the money he did raise, the website reported.

Incidentally, Bernie Sanders raised more than a fifth of his money from California — about $17 million in checks over $200. People in San Francisco’s Mission District kicked in more than $255,000 — the single biggest total of any ZIP code in the U.S.

Final flings: As barnstorms go, Bill Clinton’s final sprint through five Bay Area cities Monday was about as cheap a swing as you will find in national politics.

In both Oakland and Antioch, Clinton addressed crowds from the back of a rented pickup truck. In Hayward, he stopped at City Hall, and in Richmond, an art center.

The final stop — and probably the most expensive — was the Joseph Lee Recreation Center in San Francisco’s Bayview, which cost the campaign $1,351 for use of the gym and staff.

Meanwhile, across town at Crissy Field, Bernie Sanders wrapped up his campaign with a rally and concert featuring Dave Matthews.

The Sanders event drew thousands, and park police were needed to direct traffic and the like. But Sanders got the site for free.

“It was considered free speech and was given a First Amendment permit,” said Howard Levitt, spokesman for the Golden Gate National Recreation Area.

Levitt said he could not remember the last time the Presidio had been used for a political rally, but he did recall the last First Amendment permit that the GGNRA issued.

“It was for the Mighty Mutt March” — an April protest over proposed restrictions on where people can walk their dogs.

San Francisco Chronicle columnists Phillip Matier and Andrew Ross appear Sundays, Mondays and Wednesdays. Matier can be seen on the KPIX-TV morning and evening news. He can also be heard on KCBS radio Monday through Friday at 7:50 a.m. and 5:50 p.m. Got a tip? Call (415) 777-8815, or email matierandross@sfchronicle.com. Twitter: matierandross