Popular former Gov. Brian Schweitzer (D) will not run for the Senate in Montana, according to the Associated Press.

Schweitzer’s decision comes as a major blow to Democrats’ efforts to keep the seat to be vacated next year by retiring Sen. Max BaucusMax Sieben BaucusOvernight Defense: McCain honored in Capitol ceremony | Mattis extends border deployment | Trump to embark on four-country trip after midterms Congress gives McCain the highest honor Judge boots Green Party from Montana ballot in boost to Tester MORE (D).

“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."

Democrats saw Schweitzer, 57, as their best shot at the seat in a state that leans Republican. Termed out after eight years as governor, Schweitzer — who ran for the Senate in 2000 — remains popular in the state. 

His approval rating was 56 percent in a February survey from Public Policy Polling, with 37 percent disapproving.

Last month, Sen. Jon TesterJonathan (Jon) TesterPoll: McConnell is most unpopular senator Trump nominees meet fiercest opposition from Warren, Sanders, Gillibrand The Hill's 12:30 Report: Pelosi looks to squash fight with progressives MORE (D-Mont.) said he would "bet the farm" on Schweitzer running. 

After a hard look at the race, Schweitzer said he didn’t want to trade his jeans and bolo ties for the buttoned-up Capitol Hill uniform.

"I don't want to leave Montana,” he said. “This is my home, not Washington, D.C. I don't want a job where I have to wear a suit and my dog isn't welcome."

“I know what a tough choice this was for Brian, and I respect that he made the best decision for himself and his family. No one understands the pull of wanting to be home in Montana better than I do,” Baucus said in a statement. “Brian was a great Governor, and there’s no question he would have made an excellent candidate. But we still have a wide field of qualified Montana Democrats who can run - and win - this seat.”

Baucus and Schweitzer have long had a strained relationship. Earlier this year, Schweitzer touted a poll showing that he could defeat Baucus in a primary.

Without Schweitzer in the race, Butte native and Emily's List Executive Director Stephanie Schriock, Montana schools Superintendent Denise Juneau and Monica Lindeen, the state's insurance commissioner, are potential Democratic candidates.

None are seen as formidable as Schweitzer.

"We remain confident that Democrats can hold the Montana seat and the overall math still favors Democrats next year," Democratic Senatorial Campaign Committee Executive Director Guy Cecil said in a statement. "Only three Democratic incumbents have lost reelection in the last decade. Our incumbents are positioned to win, we've already recruited a strong challenger to Mitch McConnellAddison (Mitch) Mitchell McConnellThe Hill's Morning Report: Trump walks back from 'send her back' chants GOP rattled by Trump rally Third Kentucky Democrat announces challenge to McConnell MORE and Republicans have failed to expand the senate map into blue and purple states."

The National Republican Senatorial Committee pounced after the decision, saying it was based on Schweitzer’s recognition that not even he could win in 2014.

“Just two days ago, Senate Democrats were quoted promising Brian Schweitzer tremendous resources to get in the race,” NRSC Communications Director Brad Dayspring said in a statement. “We did our homework and there was a lot of rust under Schweitzer's hood — a LOT of rust.”

The committee had launched an aggressive opposition research campaign targeting Schweitzer as part of the GOP’s efforts to win the seat.

U.S. Rep. Steve Daines (R-Mont.) is a top GOP candidate for the seat.

Daines may be more likely to run now that he has an easier path to the seat, though a source close to him insists Schweitzer's decision won't affect his own decision-making process.

"My focus is fixed on serving the people of Montana and doing the job they sent me to do. 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," Daines said in a statement.

--Cameron Joseph contributed to this report.

--This report was originally published at 11:45 a.m. and last updated at 2:29 p.m.