ST. PAUL — A former Republican White House lawyer in President George W. Bush’s administration and a prominent critic of President Donald Trump announced Monday that he’ll run as a Democrat for Al Franken’s Minnesota Senate seat.

Richard Painter said he plans to challenge Sen. Tina Smith, who was appointed to Franken’s seat after his January resignation, in a Democratic primary in August. Painter served as chief White House ethics lawyer from 2005 to 2007 in the Bush administration but has been a visible critic of Trump since his 2016 election, making frequent appearances on CNN, MSNBC and other cable TV networks.

Explaining his party swap, Painter said it’s clear to him there’s no space for a Republican who opposes the president. And while he rattled off a handful of Democratic positions such as single-payer health care and blocking mining projects in northeastern Minnesota, he said he wouldn’t spend campaign money to attack Smith.

He made clear that the primary motivation behind his Senate bid was the president, whose policies he called “autocratic.”

“I’m running against Donald Trump and every one of his collaborators in the Republican Party,” he said. “I’m going to put my country first. This isn’t about party.”

The special election for Minnesota’s Senate seat was triggered in January, when Franken left office more than a month after a slew of women came forward with accusations of sexual misconduct. The winner of the election will serve out Franken’s term ending in 2020.

Democratic Gov. Mark Dayton appointed Smith — then his lieutenant governor — to take the seat, and Smith is running this fall to keep it. State Sen. Karin Housley is running unopposed for Republicans.

Painter had been exploring a run since early April, when he first expressed interest but was undecided on whether he’d run as a Republican, Democrat or an independent candidate. He works as a law professor at the University of Minnesota.

He said he ultimately decided against an independent bid out of concern he would poach votes from Smith and ultimately hand the election to a Republican.

“I can’t be a spoiler, not this year,” he said. “Our democracy is at stake.”