Richard Painter, a former ethics chief during former President George W. Bush's administration who has become a high-profile critic of President Trump Donald John TrumpOmar fires back at Trump over rally remarks: 'This is my country' Pelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare Trump mocks Biden appearance, mask use ahead of first debate MORE, formally announced Monday that he would be running for a Minnesota Senate seat as a Democrat.

Painter, a frequent presence on cable news who has become well-known for his broadsides against Trump, said his former party had essentially left him.

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"I tried to salvage the situation with the Republican Party for a long time," he said at a press conference announcing his candidacy. “We’ve had for decades a departure from the fundamental values of the Republican Party and from America.”

The Star Tribune first reported Sunday that Painter had filed paperwork with federal elections officials to run against Sen. Tina Smith (D-Minn.) in the Democratic primary.

Smith was appointed to fill former Sen. Al Franken Alan (Al) Stuart FrankenGOP Senate candidate says Trump, Republicans will surprise in Minnesota Peterson faces fight of his career in deep-red Minnesota district Getting tight — the psychology of cancel culture MORE’s (D-Minn.) seat after he resigned under the cloud of sexual harassment allegations.

Painter, 56, at the press conference announcing his bid, called for people to “stand firm against what President Trump is threatening to do to our country.”

He argued that Trump is changing the GOP.

“Republicans insist that if you’re going to run for political office, you have to be loyal to President Trump,” Painter said. “This is wrong. This is not the America I want to live in. This is not the America I want my children to live in.”

State Sen. Karin Housley (R-Minn.) is running for the GOP nomination for the seat.

The winner of November's election will serve out the remainder of Franken’s term, which runs though 2020.