Billionaire Tom Steyer pledged on Monday to spend $30 million through his NextGen America organization to help Democrats win congressional seats in the 2018 midterms.

Steyer, who has boosted his national profile with a recent series of television ads calling for the impeachment of President Donald Trump, said he won’t run for office himself in 2018. “That’s not where I can make the biggest difference,” he said at a news conference in Washington.

The mega-donor helped create what began as the NextGen Climate network in 2012 after leaving Farallon Capital Management, the hedge fund he co-founded, to devote himself to conservation. In 2016, the group focused on mobilizing young voters around climate change policies. Steyer said he’ll use his money similarly in 2018.

Steyer was the biggest individual political donor in the 2016 election, and his decision removes some uncertainty for California’s races for governor and U.S. Senate. When asked about his plans after the midterms, Steyer declined to rule out a potential presidential bid in 2020.

This article was provided by Bloomberg News.