Before and after his presidential run, Bullock insisted the Senate didn't interest him. But Democrats continued to recruit him into the race ahead of Monday's filing deadline, hoping that if he ran it would put Montana on the map and give them another opportunity to flip a seat in the uphill battle to win back the Senate.

Last month, Bullock met with former President Barack Obama in Washington and with Senate Minority Leader Chuck Schumer in Montana, clear signs that he was reconsidering his opposition to running for the upper chamber.

Several other Democrats had already entered the race, including public health expert Cora Neumann and Helena Mayor Wilmot Collins. But Bullock is likely to easily become the party's consensus choice against Daines.

“As governor, I’ve worked my level best to represent all Montanans and leave this place we call home better than I found it, and in a time when our politics is more divided than ever, we’ve been able to accomplish a great deal for the people of our state,” Bullock said in a statement. “We can’t say the same about Washington, where they can’t tackle the difficult challenges like health care costs, climate change and a changing economy or even the ones we agree on like strengthening our infrastructure, lowering drug prices and banning dark money from our federal elections."

Daines, a first-term senator and former one-term congressman, won his first Senate campaign by double digits in 2014. Daines' campaign manager, Shane Scanlon, touted the Republican's record and expressed confidence voters would send him back to Washington.

“We’re going to win this race because Steve Daines is always on Montana’s side fighting for more high-paying jobs, against big government and defending Montanans’ way of life,” said Scanlon. “From protecting our pocketbooks, veterans, public lands and Second Amendment rights, Daines always puts Montana first, and Montanans know they can always trust him to continue doing so when he’s reelected in November.”