"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 AP. "But I never wanted to be in the U.S. Senate. I kicked the tires. I walked to the edge and looked over."

Schweitzer aides did not immediately respond to a request for comment.

The popular former governor had been widely expected to run since Sen. Max Baucus (D) announced his retirement. And as recently as this week, people close to Schweitzer were under the impression that he was about to enter the race.

With his exit, Democrats must find a candidate capable of holding another open seat in a red state. They are still seeking standard-bearers in two other red states where they have retiring Democratic incumbents: South Dakota and West Virginia.

Those three seats would go a long way toward Republican efforts to reclaim the Senate majority in 2014 -- a goal that requires the GOP taking six Democratic seats.

Democrats also must defend three other red states where incumbents are seeking reelection -- Alaska, Arkansas and Louisiana -- along with swing states in Iowa and North Carolina. About the only GOP seats that could conceivably be in play at this point are red-tilting Kentucky, where Senate Minority Leader Mitch McConnell (R) faces Secretary of State Alison Lundergan Grimes (D), and an open-seat race in Georgia that is still taking shape.

Schweitzer's decision is expected to open the door for other Republicans to run, most notably freshman Rep. Steve Daines. Currently running on the GOP side are former state senator Corey Stapleton and state Rep. Champ Edmunds.

Republicans immediately sought credit for pushing Schweitzer out of the race.

“Just two days ago, Senate Democrats were quoted promising Brian Schweitzer tremendous resources to get in the race,” said Brad Dayspring, a spokesman for the National Republican Senatorial Committee. "We did our homework and there was a lot of rust under Schweitzer's hood – a lot of rust."

Guy Cecil, the executive director of the Democratic Senatorial Campaign Committee, insisted Democrats remain in position to hold their majority.

"We remain confident that Democrats can hold the Montana seat and the overall math still favors Democrats next year," Cecil said. "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 McConnell and Republicans have failed to expand the Senate map into blue and purple states."

Among other potential Democratic recruits are state Supreme Court Justice Brian Morris, state Sen. Kendall Van Dyk, state schools superintendent Denise Juneau, state insurance commissioner Monica Lindeen and Stephanie Schriock, the president of the Democratic women's group Emily's List.

Schweitzer is thought to be interested in a potential run for president in 2016. He also recent accepted the chairmanship of a major mining company in Montana.

Updated at 1:34 p.m.