A person familiar with Schweitzer's thinking tells The Fix the Democrat is leaning toward running. There had been chatter that the former governor, who left office earlier this year after two terms, might even challenge Baucus in the primary. He also is viewed as a dark horse 2016 presidential candidate, though few voters outside his home state know much about him.
Schwietzer has often brushed off talk about joining Congress, even as he's been viewed as his party's top potential recruit. "I am not goofy enough to be in the House, and I'm not senile enough to be in the Senate," he told the Associated Press last year. It wouldn't be Schweitzer's first bid for the Senate. He ran in 2000, losing a close race to Republican Conrad Burns.
Schweitzer raised eyebrows earlier this year when he posted a poll to his Facebook page that suggested he'd be a stronger candidate than Baucus, with whom he has clashed at times.
The former governor left office in sound political shape. Polls showed six in 10 Montana voters approved of Schweizer during his final year in office, strong standing to launch a race for the Senate. Specifically, a January 2012 Colorado College poll found 65 percent of registered voters approving and 24 percent disapproving of Schweitzer. The poll was conducted Jan. 2-7, 2012 among 400 registered voters by Public Opinion Strategies (R) and Fairbank, Maslin, Maulin, Metz & Associates (D).
Another plus for Schweitzer: He would not have the baggage of having to defend votes for President Obama's priorities like Baucus would have had to do.
Most outward indications were that Baucus was planning to run, which means that Schweitzer could be looking at a situation he wasn't anticipating. Baucus was raising money at a healthy pace and had even been airing ads as early as last year.
A former Baucus aide who was in touch with the senator's staff on Tuesday said Baucus opted to step aside for reasons more personal than electoral. Baucus wanted to spend more time with his family, said the former aide, who spoke on the condition of anonymity to provide a candid take.
Karl Struble, a Democratic strategist who used to work for Schweitzer, said he thinks the former governor will make a bid for Baucus's seat. "He's the best chance Democrats have," Struble said.
At this point, the Republican field is populated by two low-profile current and former state legislators. Former state senator Corey Stapleton, who lost the 2012 GOP gubernatorial nomination, is joined by state Rep. Champ Edmunds, who looks to be carving out ground at the rightward end of the field.
There is plenty of time for Republicans to coalesce around a candidate. Look for Rep. Steve Daines (R), 2012 GOP nominee Denny Rehberg, and state Attorney General Tim Fox (R) to be mentioned as potential candidates as the open race unfolds.
“Steve is humbled by the calls and e-mails encouraging him to consider a run for the Senate, and given today’s announcement by Senator Baucus, he is giving it serious and thoughtful consideration,” Daines spokeswoman Alee Lockman said.
Reached Tuesday morning, a spokesman for Fox said the attorney general is happy in his current position, and looks forward to continuing his service.
"Montanans elected Tim Fox to serve as their attorney general. He looks forward to serving them in that capacity and working hard to make Montana safer and more prosperous in the months and years to come," said Fox spokesman John Barnes.
The conservative tilt of Montana should be enticing for potential GOP candidates. Remember, we're talking about a state where Mitt Romney won by double-digits in 2012, so even if Democrats get Schweitzer, this is going to be a challenging race for them.
On the Democratic side, one more name to keep an eye on is EMILY's List head Stephanie Schriock. She is a Montana native who once worked for Sen. Jon Tester (D), who defied the odds to win reelection last year. Schriock's Montana roots and relationships with national donors make her an intriguing potential candidate to watch.
Scott Clement and Aaron Blake contributed to this report. Clement is a pollster with Capital Insight, the Washington Post's independent polling group.
Updated at 12:24 p.m.