From left, President Bill Clinton, Vice President Al Gore, Hillary Clinton, House Minority Leader Richard Gephardt and White House Chief of Staff John Podesta after the House voted to impeach the president on Dec. 19, 1998. (Greg Gibson/Associated Press)

Former vice president Al Gore will start campaigning for Democratic nominee Hillary Clinton, according to individuals briefed on the plan, in an effort to mobilize young voters who see climate change as a key issue.

The decision by Gore to plunge into the campaign during the final weeks shows the extent to which Democrats remain concerned that Clinton has yet to connect with many millennials, some of whom are backing third-party candidates this year. The former vice president, a climate activist, will speak about not just Clinton’s plan to address global warming, but also the idea that voting for an independent presidential candidate could deliver the White House to Republicans in the same way that Ralph Nader’s candidacy helped undermine his presidential bid in 2000.

CNN first reported Gore’s plans Monday evening.

Gore first endorsed Clinton’s candidacy in late July in a three-part tweet, writing, “Given her qualifications and experience and given the significant challenges facing our nation and the world, including, especially, the global climate crisis, I encourage everyone else to do the same.”

But he has stayed largely on the sidelines during the campaign since then, in part because the two politicians have been distant since the end of Bill Clinton’s time in office. Their relationship became strained for many reasons, including the fact that Gore distanced himself from the two-term president in the wake of his affair with Monica Lewinsky, and they competed for Democratic donors when they were both running for office in 2000.

Support from millennials could play a critical role in some swing states: A new poll from Christopher Newport University showed Clinton received a boost in Virginia as some young voters drifted back from third parties to the Democrat. And while climate change remains a low priority for most voters, it ranks higher among millennials.

Clinton’s campaign has relied heavily on independent groups to mobilize younger voters on environmental issues this cycle. NextGen Climate–which is bankrolled by billionaire Tom Steyer–has played a prominent role, establishing operations on college campuses in states such as New Hampshire, Pennsylvania, Ohio, Illinois, Nevada and Iowa in an effort to influence not only the presidential race but key Senate contests.

On Wednesday NextGen Climate announced that Jessica Williams, co-host of the podcast “2 Dope Queens” and a former cast member of The Daily Show, will host a comedy tour the group is sponsoring in Pennsylvania, Ohio and North Carolina. At each stop the group’s organizers will discuss the issue climate change with members of the audience, and offer them the opportunity to register to vote.

In a recent interview with the Post, Steyer said that the group’s polling shows that once millennials learn about Clinton’s and Donald Trump’s position on climate change, her approval level with these voters increases significantly.

“We do not think you can [expletive] millennials,” he said. “If you can be honest, and transparent, and consistent on the issues they care about, they come around. This is a passionate generation.”

Gore’s camp was working out the details of his participation with Democratic National Committee interim chair Donna Brazile on Tuesday, according to one individual who spoke on the condition of anonymity because the decision had not been announced yet.

Gore’s office declined to comment for this story on Wednesday.

Wayne Skinner, a longtime Democratic advance staffer, hailed Gore’s decision to stump for Clinton on his Facebook page Monday night, writing: “Let’s get him to Florida ASAP! We are #StrongerTogether. No better validator and reminder that EVERY SINGLE vote counts and matters. #RememberBushVGore #Gore2000 #Florida #RalphNader#SpoilerVote.”