WASHINGTON - Republicans received a boost in their attempt to win back the majority in the Senate next year when a former Democratic governor bowed out of Montana's open Senate race, a development that could further hamper President Obama's agenda.

Former Montana Gov. Brian Schweitzer said Saturday he would not run for the seat of retiring Democratic Sen. Max Baucus in 2014, dealing a blow to Democrats who considered the popular ex-governor their best chance of keeping the office.

Republicans need to pick up six seats to recapture the Senate majority and are trying to take advantage of geography and history in their quest. Democrats must defend 21 seats, including seven in largely rural states that Obama lost in 2012, and the party that controls the White House typically loses seats during the midterm elections of a second-term president.

"For the first time in a couple of years, you can see the Democratic majority has never been on shakier ground," said Rob Jesmer, a Republican strategist and former executive director of the National Republican Senatorial Committee.

Democratic retirements in Republican-leaning states like West Virginia and South Dakota have given Republicans an advantage. The GOP has recruited popular Republicans for those seats, Rep. Shelley Moore Capito of West Virginia and former South Dakota Gov. Mike Rounds.

In Montana, Republicans hope to persuade Rep. Steve Daines or perhaps former Gov. Marc Racicot to mount a campaign, which could give them an edge in three of the six states they would need to win the majority.