Updated 1:15 p.m. | Former Montana Gov. Brian Schweitzer, a Democrat, will not run for the Senate, according to an Associated Press report.
The stunning decision immediately puts the open seat in jeopardy for Democrats and makes the party's hold of the majority in 2014 a heavier lift.
"I love Montana. I want to be here. There are all kinds of people that think I should be in the U.S. Senate," Schweitzer told the AP. "But I never wanted to be in the U.S. Senate. I kicked the tires. I walked to the edge and looked over."
Just a few days ago, he was expected to run. Earlier in the cycle, operatives believed he would have been the frontrunner if he sought the seat given his popularity as governor.
Republicans, who had already begun building an opposition research file on the former governor, are now even more optimistic of picking up the seat.
"We did our homework and there was a lot of rust under Schweitzer's hood – a LOT of rust," National Republican Senatorial Committee spokesman Brad Dayspring said. "Republicans will have a great candidate to ensure the Treasure State equal representation in the Senate."
The GOP field currently includes former state Sen. Corey Stapleton and state Rep. Champ Edmunds. But it's now highly likely that they will face stiff competition for the nomination.
Rep. Steve Daines, a freshman Republican who briefly ran for Senate last cycle, is considering a run and would be favored to win the primary. He raised $433,000 in the second quarter and had nearly $600,000 in cash on hand as of June 30.
"My focus is fixed on serving the people of Montana and doing the job they sent me to do," Daines said in a statement on Saturday. "I will continue to give this decision the consideration it deserves, and am still taking time to talk with my family and the people of Montana about how I can best be of service to our state."
This is an open-seat race to replace retiring Democratic Sen. Max Baucus, with whom Schweitzer had a frosty relationship at best. Schweitzer also had not spoken with Democratic Sen. Jon Tester since before the 2012 elections.
Despite the GOP's opposition file and Schweitzer's penchant to rub even those within his party the wrong way, party insiders still felt he would be formidable in a general election. His departure from the scene leaves Democrats turning elsewhere.
A name mentioned months ago was EMILY's List President Stephanie Schriock, a Montana native and former top aide to Tester. She said in May that she would consider the race.
In reaction to Schweitzer's exit, the Democratic Senatorial Campaign Committee pushed back on the notion that the majority is in further jeopardy. DSCC Executive Director Guy Cecil said in a statement that the party remains "confident that Democrats can hold the Montana seat and the overall math still favors Democrats next year."
Tester, for one, remains confident that the party will find someone else for Montana's competitive race.
"Running for the U.S. Senate is a big decision for a potential candidate and their family," Tester said in a statement. "Just because this year wasn't the right time for Brian to run doesn't mean we don't have great Montana Democrats who are willing to run and capable of winning the seat."