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 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 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 own 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 ever 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.

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