In a move that stunned Washington, Sen. Olympia Snowe (R) of Maine announced Tuesday that she won't run for a fourth term in the Senate for a simple reason: gridlock.

"I am well prepared for the electoral battle, so that is not the issue," said Senator Snowe, who won her last race with 74 percent of the vote. “However, what I have had to consider is how productive an additional term would be.

“Unfortunately, I do not realistically expect the partisanship of recent years in the Senate to change over the short term,” she added.

An iconic Senate moderate, Snowe often cast crucial votes in a closely divided Senate, forcing Republicans to take steps to curb the federal deficit, even when the core GOP issue of tax cuts was involved. More than once, her party's leaders had to rein in the scope of proposed tax cuts or to find offsetting sources of income to win her vote.

She also helped organize the bipartisan Gang of 14 to preserve the minority's right to filibuster judicial nominations – even though Democrats were in the minority at the time.

Dubbed the “girls from Maine” by antitax activist Grover Norquist, Snowe and Sen. Susan Collins ranked as the two most liberal Republicans, according to a Feb. 25 survey by the National Journal. Despite the grumbling that Snowe is a RINO, or "Republican in name only," conservative activist groups weren’t out to topple her – a move that would risk handing the seat to a Democrat.

“She was not at all facing a tough race, and that’s what makes her decision so perplexing,” says Jessica Taylor, senior analyst at the Rothenberg Political Report in Washington. “This was rated a safe Republican seat – a seat neither party expected to spend much money on."

“It’s a huge break for Democrats," says Ms. Taylor. "This is now one of the races that Republicans will have to spend money in. It’s a pick-up that Democrats would need. It plays a crucial role in determining who controls the Senate next year."

The Senate Democratic Campaign Committee trumpeted Snowe's announcement as an immediate opportunity to pick up a seat in a year when Republicans need to win four Senate races to take control of chamber.

“Maine is now a top pick-up opportunity for Senate Democrats," said DSCC spokesman Guy Cecil, in a statement. "Democrats not only hold a strong registration advantage in the state, but this is a state that the president won by 17 points in 2008 and will likely win by a significant margin this year as well.”

With the filing deadline is only two weeks away, it’s not going to be easy to come up with a strong candidate.

Snowe "did not face a difficult race, and it’s too soon to say whether this is now beyond the GOP’s reach,” says Jennifer Duffy, who analyzes Senate races for the Cook Political Report. “Democrats didn’t have a strong candidate in the race, so they are searching, too.”

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Two-term Rep. Chellie Pingree (D) of Maine – the most likely prospect for Democrats – called the next election “critical to the future of our working families around the country.”

“In the coming days I will carefully consider how I can best serve the people of Maine,” Congresswoman Pingree said in a statement on Tuesday. She will be holding a press conference when she returns to Portland on Friday, according to the release, to further discuss her plans.