Democratic Montana Gov. Steve Bullock will run for U.S. Senate, a senior adviser said, taking on incumbent Republican Sen. Steve Daines.

Bullock, who had embarked on a failed bid for the 2020 Democratic presidential nomination, previously said he was not interested in the Senate but changed his mind after meetings with former President Barack Obama and Senate Minority Leader Chuck Schumer, D-N.Y.

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Bullock is expected to file paperwork Monday to officially run for the seat that, before Daines, had been held for more than 35 years by Democrat Max Baucus. Baucus left office to serve as U.S. ambassador to China under the Obama administration.

A victory over Daines would help Democrats in their quest to take control of the Senate. The National Republican Senatorial Committee (NRSC) was quick to oppose Bullock's decision and support Daines.

"Steve Bullock ran his administration like a frat house and acquiesced to the far left during his comical presidential run, not exactly the strongest way to launch a Senate campaign," NRSC spokesperson Nathan Brand said in a statement. "Bullock supported impeaching President Trump, wants to ban guns, and ignore illegal immigration. One can see why, given their visceral disdain for the Montana way of life, Washington Democrats rolled out the red carpet for Bullock."

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By contrast, Brand said, "Steve Daines always puts Montana first, is a proven leader and a strong ally for President Trump."

President Trump has also expressed support for Daines and has met with him at the White House. Bullock, however, was the only Democratic governor in the U.S. who was reelected in a state that voted for Trump in the 2016 election.

The Associated Press contributed to this report.