While Republicans have been wrestling with an aging demographic for some time, analysts say the unpopular actions of President Trump are pushing away a new generation.
"One of the biggest challenges facing the Republican Party in Virginia and nationwide is that the Republican Party has become toxic to a lot of young voters," said Bob Holsworth, a longtime Virginia political scientist. "I think Trump has exacerbated a trend that was emerging, and it has become very problematic."
Northam defeated Republican Ed Gillespie by an overall margin of 54 percent to 45 percent, as certified this week by the state Board of Elections.
Young voters — who are often among the least-engaged, especially in a nonpresidential election — had a turnout rate of 34 percent, according to an analysis of exit polling by a group at the Tisch College of Civic Life at Tufts University. That's up from 26 percent in the 2013 governor's race and double the youth turnout in 2009.
And that surge of millennials was a windfall for Democrats: Sixty-nine percent of those voters supported Northam, vs. 30 percent for Gillespie.
The same trend held when measured in different ways. In precincts adjacent to college campuses, turnout was up 8 points over 2013 and Northam won 72 percent of the vote, according to an analysis by the nonpartisan Virginia Public Access Project.
In precincts not adjacent to colleges but where voters age 40 and younger make up more than 60 percent of the population, turnout was up 7 points and Northam won more than 81 percent of the vote, the VPAP analysis found.
Virginia's results suggest that the rise of young voters is a serious problem for Republicans nationwide.
Recent studies by the Pew Research Center found that millennials — generally defined as those age 18 to 35 in 2016 — are fast becoming the largest single bloc in the electorate. By 2020, there will be more millennial and Gen X voters than baby boomers.
And millennials have routinely polled as more left-leaning than other segments of the population.
The only other statewide election this year was in New Jersey, where Democrat Phil Murphy was a prohibitive favorite for governor. Young voters didn't surge in New Jersey — turnout of 18 percent was similar to the past two governor's elections there — but 73 percent of them voted for the Democrat, according to the Tufts analysis.
"Young people have increasingly moved away from the Republican Party, given its perceived status as being the anti-immigrant party and not being tolerant of alternative lifestyles," said Mark Rozell, dean of the Schar School of Policy and Government at George Mason University.
"I find even with my students who are Republican-leaning, on social or cultural issues they're very libertarian for the most part," Rozell said. "As long as the Republican Party is seen as not embracing or accepting people as they are, be they gay, transgender, immigrants and the like, that's a big turnoff to young voters these days."
The Virginia results, he said, are "a wake-up call to the Republican Party about the way things are going to go for them next year" in congressional races across the country.
Some Republicans get the message. David Ramadan, a former Republican delegate to the General Assembly who supported Gillespie for governor, said the youth turnout "is one number that certainly has popped out. I'll summarize that with one word for the GOP: trouble."
Ramadan has long warned that his party has a problem wooing minority voters. "But this election shows that it's not just a minority problem, it's a youth problem" — and even broader than that, he added.
Those problems were accelerated this year by "the Trump effect," he said.
Trump is extremely unpopular in Virginia, with a 34 percent approval rating in a recent Washington Post-Schar School poll. And his rhetoric appeared to influence Gillespie to shift his campaign toward issues of illegal immigration, Latino street gangs and support for Confederate statues.
That was a losing formula for wooing young voters, Ramadan said. Millennials care about reducing student debt, finding a comfortable job and being able to have an easy commute to work, he said.
"Unless Republicans get back to mainstream issues instead of sanctuary cities and Confederate statues, we're going to lose elections," he said.
Republican pollster Gene Ulm said he was most concerned about "young suburbanites" in the Virginia results. "We basically had 300,000 people show up who don't normally vote in a gubernatorial election," he said. "We won 85,000 of them and lost the other 215,000."
There was one silver lining: Exit polls show Gillespie won about 12 percent of the African American vote, higher than most recent GOP candidates.
By Ulm's calculation, that would roughly translate to nearly 63,000 votes — a small number but far more than any Republican statewide candidate in recent years.
"Ed Gillespie got more black votes when he lost than the last Republican governor got when he won," Ulm said, referring to former governor Bob McDonnell, who got fewer than 29,000 black votes in 2009 by his reckoning.
But the overall trend of young, white and suburban voters is a bad sign for the party going into next year's congressional races, he said. "That affects certainly Barbara Comstock," he said, referring to the Northern Virginia congresswoman, a Republican whose district went for Northam. "But there's a lot of other districts in America where that's going to play a role," Ulm said.
There are a host of progressive and grass-roots groups aiming specifically to get a large turnout of young voters for Democratic candidates, and they were taking credit for the big showing.
"At least three voter registration and mobilization groups had a presence on our campus, and there may have been more at some of the bigger campuses," said Quentin Kidd, a political scientist at Christopher Newport University in Newport News.
At Virginia Tech, in fact, the progressive group NextGen Virginia staged a petting zoo to get students to come out to vote. The group spent $3.3 million statewide and knocked on 350,000 doors as part of its efforts.
After Northam's win, NextGen crowed in a letter to supporters that "it's clear young people and people of color made the difference in this historic election . . . If anger at Donald Trump and the GOP continues to drive turnout up through the 2018 election cycle, when young people will be the largest bloc of eligible voters, we could be looking at a progressive wave powered by millennial voters."
Kidd said evidence of the youth trend's staying power can be found in the crop of new Democrats elected to the General Assembly, who are far younger — and more female — than the current legislature.
"Half these new delegates are probably millennials," he said. "The impact of the millennial generation on the electorate is going to be pretty profound, and Democratic candidates tend to be the beneficiary of this far more than Republicans are."