A deep-pocketed environmental group aligned with Hillary Clinton will blanket 1.1 million households in battleground states with mailers warning millennials that a vote for a third-party candidate only helps Donald Trump, the group told NBC News.

The League of Conservation Voters Victory Fund plans to spend $2.6 million before Election Day, most of which will go towards their efforts to prevent Libertarian Nominee Gary Johnson and Green Party nominee Jill Stein — polling at about 7 percent and 3 percent, respectively — from siphoning votes away from Clinton.

"There are high stakes for young voters in this election, including the opportunity to meet the climate crisis head-on, and they overwhelmingly dislike Trump. But some may still be leaning towards a third-party candidate instead of Hillary," said LCV National Campaigns Director Clay Schroers. "This is a group of young people who don’t want to risk a Trump presidency, and it’s important that they know that a vote for anyone but Hillary is a vote for Trump."

Millennials are crucial to Clinton’s coalition, but polls show that while they strongly dislike Trump, they’re far more interested in third-party candidates than voters overall.

LCV, which plans to spend more than $40 million this year, also has a large canvassing program and joins other groups, like MoveOn.org, in trying to curb the Johnson and Stein vote.