Hedge fund manager Robert Mercer may be the most influential political donor of the Trump era. Though fellow hedge fund manager Tom Steyer and casino mogul Sheldon Adelson have given far more money, their investments were not as well targeted.

Mercer, who’s net worth is unknown, was Donald Trump’s biggest donor ahead of the 2016 election, according to the Center for Responsive Politics. Along with his wife Diana, Mercer gave a total of $15.5 million, of which that vast majority went to a super PAC to support Trump called Make America Number 1. Originally a computer scientist, Mercer earned most of his money at the hedge fund Renaissance Technologies, which takes an almost entirely quantitative approach to their investing.

Along with donating to Trump, Mercer was an early investor in Breitbart News, an outlet supportive of Trump’s anti-immigration, nationalist agenda. The Mercers also started Cambridge Analytica, a data-focused political consultancy that worked for the Trump campaign. (Cambridge Analytica illegally harvested data on 50 million Facebook users and has since shut down.)

Add it all up and Mercer has been uniquely important to Trump’s ascent.

Mercer is not contributing quite as much to the 2018 midterm effort, but at about $6 million he is still among the 15 biggest donors to conservatives in this election cycle. The Center for Responsive Politics found that of Trump’s biggest donors in 2016, only Bernard Marcus, one of the co-founders of Home Depot, and his wife Billi are giving more in 2018 than they did in 2016.

Mercer’s giving in 2018 is more diverse, with over 40 groups receiving some money. The biggest beneficiaries of his largesse have been Republican senate candidates who ran to the right of candidates supported by the party establishment, like Chris McDaniel in Mississippi ($1 million) and Kelli Ward in Arizona ($800,000). Mercer appears determined to make the Republican party ever more conservative, and he continues to make investments tailored to make that happen.