A billionaire Wall Street executive and Donald Trump adviser has become the biggest donor to the super PAC backing Maine Republican Sen. Susan Collins.

Blackstone Group CEO Steve Schwarzman gave another $500,000 earlier this month to help Collins in a race that could have major financial implications for his firm. In all, Schwarzman has funneled $2 million to the 1820 PAC, a single-candidate super PAC backing her campaign.

Schwarzman has a personal stake in the outcome of Senate elections: as the Daily Poster first revealed, his private equity firm reported a negative effective tax rate last year despite generating billions in profits. Blackstone’s conversion was prompted by Republicans’ 2017 tax law, which slashed the federal corporate income tax rate from 35 percent to 21 percent.

Due to its conversion, the firm’s future profits are tethered to the federal corporate tax rate, and could therefore be complicated if Democrats manage to win control of the Senate and the presidency and follow through on Vice President Joe Biden’s pledge to raise the rate.

Schwarzman has emerged as one of the Senate GOP’s biggest donors this election cycle, contributing a staggering $20 million to the primary super PAC working to elect Senate Republicans.

The Maine Senate race could decide which party controls the Senate next year. The election will likely be close — state House speaker Sara Gideon currently leads Collins by 4.2 percent in the latest average of polls compiled by RealClearPolitics.

While Collins was initially undecided on the 2017 tax law, she ultimately voted for the legislation and helped secure its passage. Gideon’s campaign has criticized Collins for her vote, saying it “benefited corporate donors, not Mainers.”

Blackstone executives have also been a top source of direct cash to Collins’s campaign, according to OpenSecrets. Schwarzman hosted a fundraiser for Collins in New York last October.