Updated 9:58 a.m. ET, 7/15
(CNN) - In a surprising twist for the 2014 midterm elections, former Democratic Gov. Brian Schweitzer of Montana will not run for the U.S. Senate as expected, according to a source familiar with his decision.
Schweitzer was widely considered to be the Democratic frontrunner to campaign for the seat and keep it in Democrats' hands after the current holder, Sen. Max Baucus, retires at the end of his sixth term.
After Baucus' retirement announcement in late April, Schweitzer said he was thinking about a Senate bid, a senior Democratic official told CNN at the time.
The folksy former governor, known for his signature bolo tie, had strong approval ratings in office and was elected to his second term in 2008 with 65% of the vote.
Schweitzer told the Associated Press on Saturday that he didn't want to leave Montana for Washington.
He pointed to his new position as board chairman of the Stillwater Mining Company in Montana, saying he has “taken on a new life” at the company. His election to the position in May sparked speculation that it may affect his thoughts on a Senate run.
But a source familiar with his decision not to run said Schweitzer was beginning to recognize what senior Democrats in Washington had feared.
“The research book was getting worse by the minute,” the source said.
Schweitzer’s name has popped up in recent reports this month regarding his ties to a political nonprofit group and questions over its disclosure practices.
Schweitzer was a vocal opponent of a the 2010 Supreme Court ruling that paved the way for super PACs, which are allowed to raise and spend unlimited amounts of money as long as they don't coordinate with campaigns, and lambasted secret money campaigns.
But the former governor denied on Saturday that the recent controversy affected his decision.
"This isn't my first rodeo," Schweitzer told the AP.
The Democrats control the Senate 54-46 (with two independents caucusing with the party), but they need to defend 21 of the 35 seats up for grabs in November 2014.
For their part, Republicans welcomed Schweitzer's announcement. "This news is a pivotal, sea-chage moment in our efforts to earn back the majority in the Senate," said Brad Dayspring, spokesman for the National Republican Senatorial Committee.
The NRSC had already launched attacks on Schweitzer in preparation for his bid, considering him a potentially vulnerable Democrat in a state with large swaths of Republican voters. He's also been considered a potential dark horse in the Democratic presidential primary in 2016.
In their effort to retake the majority, Republicans are battling to win back seats held by Democrats in red states like Montana, West Virginia, South Dakota, and Arkansas. With Schweitzer off the ticket, that puts Montana more in play for the GOP.
But Guy Cecil, executive director of the Democratic Senatorial Campaign Committee, said the party is "confident" that Democrats can hold onto the Montana seat and predicted the "overall math still favors Democrats next year."
"Only three Democratic incumbents have lost reelection in the last decade," he said in a statement. "Our incumbents are positioned to win, we've already recruited a strong challenger to Mitch McConnell and Republicans have failed to expand the senate map into blue and purple states."
Potential Democratic names floating around for the Montana seat include EMILY’s List President Stephanie Schriock, Montana Supreme Court Justice Brian Morris, state Sen. Kendall Van Dyk, Montana public schools Superintendent Denise Juneau and state Insurance Commissioner Monica Lindeen.
Schweitzer's decision will no doubt leave some disappointed. The liberal Progressive Change Campaign Committee, a grass-roots group that in part cut its teeth on Elizabeth Warren's successful Senate campaign in Massachusetts, had been aggressively pushing a Schweitzer run.
Almost immediately after Baucus announced in April that he would not run for re-election, the group went ahead with its Draft Brian Schweitzer campaign. It has since raised $35,000, with $10,000 raised in the first five hours. (When the group supported Warren, it raised $100,000 before she launched her campaign and $1.1 million overall.)
More than 22,000 people had signed their support to the effort to draft Schweitzer, with many of them pledging to donate or volunteer if he decided to run.
"We are urging him to reconsider," the group's co-founder, Adam Green, told CNN.
– CNN Senior Producer Kevin Bohn contributed to this report.