Republican Senator Olympia Snowe, a three-term veteran from Maine, will not stand for re-election this fall. Snowe, one of the handful of moderates left in the Senate, said she is tired of the gridlock that has paralyzed Congress.

Snowe, 65, was one of just three Republicans to support President Obama's stimulus package in 2009. Although she voted against the final health care bill in 2010, she was the only Republican Senator to vote for any version of the bill (she supported it in committee).

Her retirement is a big blow to Republican hopes of taking control of the Senate. With her seat almost certain to be picked up by a Democrat, Republicans would need to pick up a total of four seats (three if a Republican wins the White House) - and not lose any of their own vulnerable seats like Scott Brown's seat in Massachusetts - in order to take control of the Senate.

In a paper statement announcing her retirement, Snowe said she does not "realistically expect the partisanship of recent years in the Senate to change."

Snowe's office said she will hold a news conference in Portland, Maine, in order to further discuss her decision when she returns to her home state on Friday.

ABC's Sunlen Miller contributed to this post.

Snowe's Full Statement:

"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.

"With my Spartan ancestry I am a fighter at heart; and I am well prepared for the electoral battle, so that is not the issue. 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. So at this stage of my tenure in public service, I have concluded that I am not prepared to commit myself to an additional six years in the Senate, which is what a fourth term would entail.

As I enter a new chapter, I see a vital need for the political center in order for our democracy to flourish and to find solutions that unite rather than divide us. It is time for change in the way we govern, and I believe there are unique opportunities to build support for that change from outside the United States Senate. I intend to help give voice to my fellow citizens who believe, as I do, that we must return to an era of civility in government driven by a common purpose to fulfill the  promise that is unique to America.

"In the meantime, as I complete my third term, I look forward to continuing to fight for the people of Maine and the future of our nation.  And I will be forever and unyieldingly grateful for the trust that the people of Maine have placed in me, and for the phenomenal friendship and assistance I have received over the years from my colleagues, my supporters, and my staff, both in Maine and in Washington."