WASHINGTON — The billionaire environmentalist Thomas F. Steyer and his political advocacy group, NextGen Climate, will spend at least $25 million on a get-out-the-vote campaign targeting young voters this year in seven mostly battleground states, the group announced on Monday.

Mr. Steyer, the single biggest political donor of the 2014 midterm election cycle, said the campaign would target at least 203 college and university campuses. He called it the largest youth voter outreach program ever undertaken by a candidate or political campaign.

Mr. Steyer spent $74 million in the 2014 midterm elections, including $67 million for NextGen Climate to reward candidates who embraced climate change as a major issue. That year, Democrats were clobbered, losing control of the Senate and falling deeper into the minority in the House. Of the seven candidates for Senate or governor NextGen Climate spent money on, three won.

Undeterred, Mr. Steyer said on Monday that the $25 million dedicated to voter outreach was a floor, not a ceiling. His group expects to press other efforts focused on electing climate-friendly lawmakers.