Richard Painter talked with staff before walking into a press conference to announce his candidacy for U.S. Senate as a Democrat. (Deb Pastner — Star Tribune/The Minnesota Star Tribune)

Richard Painter, a longtime Republican who was chief ethics lawyer for George W. Bush's White House, intends to run for the U.S. Senate in Minnesota this year as a Democrat, according to a filing he made recently with federal elections officials.

Painter, a persistent and frequent critic of President Donald Trump on national cable TV news appearances and on Twitter, is expected to announce his candidacy at a Monday news conference.

He's running for Democrat Al Franken's former seat. Franken resigned Jan. 2 in the wake of numerous sexual harassment allegations.

Gov. Mark Dayton appointed Lt. Gov. Tina Smith to be his successor. That seat is up this fall in a special election, and Smith has said she intends to run for the right to finish the term through 2020.

Minnesota's other Senate seat, held by Democrat Amy Klobuchar, is also up this fall, although she is widely expected to win another term.

Painter, 56, a University of Minnesota professor of corporate law who is seeking elected office for the first time, lives in Mendota Heights with his wife, Karen, an associate professor of music history at the U. They moved to Minnesota in 2007 and have children.

He announced last month that he was forming an exploratory committee. At that time, he said he was unsure whether he would run as a Republican, Democrat or independent.

"I need to think about whether there's a place for me" in the GOP, he said at the time. "I'm going to be considering any and all options." He described himself as "a centrist in many ways — right up the middle." He said he has supported Democrats.

Karin Housley, a small-business owner and suburban state senator, is the only Republican to announce plans to run for the seat. She was elected to the Legislature in 2012.

Painter has said that if he runs, he would not accept donations from political action committees, super PACs or what he called "dark money" groups.

Painter was born in Philadelphia in 1961. He graduated from Harvard and then Yale Law School. He was Bush's ethics lawyer from 2005 to 2007.

He is vice chairman of Citizens for Responsibility and Ethics in Washington. That group sued Trump, alleging that he violated the U.S. Constitution's emoluments clause by refusing to sell his assets or put them in a blind trust. The case was dismissed in December.

Staff writer Judy Keen contributed to this report. Paul Walsh • 612-673-4482