AP
Montana's seat is now a little more wide open in the race to replace Sen. Max Baucus, who announced earlier this year that he would retire. Polls showed that Schweitzer, who was still popular in the red state, was the only hypothetical Democratic candidate who led potential Republican candidates.
Republicans need to flip six Senate seats in 2014 to gain back control of the body. Now three red states with retiring Democratic senators — Montana, West Virginia, and South Dakota — look like promising potential flips for Republicans, who reacted to the news with glee.
"Just two days ago, Senate Democrats were quoted promising Brian Schweitzer tremendous resources to get in the race," said Brad Dayspring, the communications director at the National Republican Senatorial Committee, in an email.
"Fact is Schweitzer looked at the race and realized he wouldn't win in light of how unpopular the Democrat agenda of higher taxes, bankrupting spending, and the ObamaCare train wreck is in Montana these days. Republicans will have a great candidate to ensure the Treasure State equal representation in the Senate."
Democrats are spinning the news by suggesting that Schweitzer was an ultimately flawed candidate. But in April, they painted Baucus' retirement as one that made them stronger in 2014 because it cleared the field for Schweitzer.
"Montana was never going to be an easy state for us to defend," one high-placed Democratic source told Business Insider. "But Schweitzer is the much stronger candidate."
A Public Policy Polling survey taken in late June showed that Schweitzer was the only possible Democratic candidate who consistently beat most of the possible Republicans. Schweitzer was viewed favorably by 54% of Montanans, compared with just 40% who viewed him unfavorably.