In a highly unexpected announcement, Olympia Snowe, the Senator from Maine, will not seek re-election, putting another seat on the table for the Democrats in the Senate and possibly shifting what was seen as a likely Republican takeover into more of a tossup.

Here’s the statement from Snowe, who didn’t really face much resistance for re-election:

After an extraordinary amount of reflection and consideration, I am announcing today that I will not be a candidate for re-election to the United States Senate.

After 33 years in the Congress this was not an easy decision. My husband and I are in good health. We have laid an exceptionally strong foundation for the campaign, and I have no doubt I would have won re-election. It has been an indescribable honor and immeasurable privilege to serve the people of Maine, first in both houses of Maine’s legislature and later in both houses of Congress. To this day, I remain deeply passionate about public service, and I cherish the opportunity I have been given for nearly four decades to help improve the lives of my fellow Mainers.

As I have long said, what motivates me is producing results for those who have entrusted me to be their voice and their champion, and I am filled with that same sense of responsibility today as I was on my first day in the Maine House of Representatives. I do find it frustrating, however, that an atmosphere of polarization and ‘my way or the highway’ ideologies has become pervasive in campaigns and in our governing institutions.

So Snowe alludes to the constant pressure on her to join her Republican colleagues on virtually every issue to block progress in the Senate, and presumably the prospect that she would have to do that for the rest of her career. This is another example of moderates being run completely out of the Republican Party.

This does jumble the picture in the Senate, as a totally safe seat for Republicans now becomes a contested seat. There’s no word on who would seek the nomination on either side, since this was unanticipated. But Democrats have several possibilities, from 2008 candidate and former Rep. Tom Allen to former House speaker Hannah Pingree. And given the low probability of a moderate coming out of a Republican primary, I would have to guess that the Democrat would be favored, given the top of the ticket race.

This is basically a gift for Democrats, who now have multiple seats to contest (in Massachusetts, Nevada and now Maine) in addition to the seats they’re defending across the country (including open seats in Nebraska, North Dakota, Wisconsin and Virginia, and tough races in Montana and Missouri).