Pennsylvania Sen. Arlen Specter today announced that he will be switching his party affiliation from Republican to Democrat and will run in 2010 as a Democrat. The Washington Post's Chris Cillizza writes:

Specter's decision would give Democrats a 60 seat filibuster proof majority in the Senate assuming Democrat Al Franken is eventually sworn in as the next Senator from Minnesota. (Former Sen. Norm Coleman is appealing Franken's victory in the state Supreme Court.)

"I have decided to run for re-election in 2010 in the Democratic primary," said Specter in a statement. "I am ready, willing and anxious to take on all comers and have my candidacy for re-election determined in a general election."

"Since my election in 1980, as part of the Reagan Big Tent, the Republican Party has moved far to the right. Last year, more than 200,000 Republicans in Pennsylvania changed their registration to become Democrats. I now find my political philosophy more in line with Democrats than Republicans."

CNN's Dana Bash called the news "seismic." CNN's Ed Henry said that the President heard about Specter's shift at 10:25 a.m. He then called Specter and said, "You have my full support and we're thrilled to have you."

Specter's full statement is below:

I have been a Republican since 1966. I have been working extremely hard for the Party, for its candidates and for the ideals of a Republican Party whose tent is big enough to welcome diverse points of view. While I have been comfortable being a Republican, my Party has not defined who I am. I have taken each issue one at a time and have exercised independent judgment to do what I thought was best for Pennsylvania and the nation.

Since my election in 1980, as part of the Reagan Big Tent, the Republican Party has moved far to the right. Last year, more than 200,000 Republicans in Pennsylvania changed their registration to become Democrats. I now find my political philosophy more in line with Democrats than Republicans.

When I supported the stimulus package, I knew that it would not be popular with the Republican Party. But, I saw the stimulus as necessary to lessen the risk of a far more serious recession than we are now experiencing.

Since then, I have traveled the State, talked to Republican leaders and office-holders and my supporters and I have carefully examined public opinion. It has become clear to me that the stimulus vote caused a schism which makes our differences irreconcilable. On this state of the record, I am unwilling to have my twenty-nine year Senate record judged by the Pennsylvania Republican primary electorate. I have not represented the Republican Party. I have represented the people of Pennsylvania.

I have decided to run for re-election in 2010 in the Democratic primary.

I am ready, willing and anxious to take on all comers and have my candidacy for re-election determined in a general election.

I deeply regret that I will be disappointing many friends and supporters. I can understand their disappointment. I am also disappointed that so many in the Party I have worked for for more than four decades do not want me to be their candidate. It is very painful on both sides. I thank specially Senators McConnell and Cornyn for their forbearance.

I am not making this decision because there are no important and interesting opportunities outside the Senate. I take on this complicated run for re-election because I am deeply concerned about the future of our country and I believe I have a significant contribution to make on many of the key issues of the day, especially medical research. NIH funding has saved or lengthened thousands of lives, including mine, and much more needs to be done. And my seniority is very important to continue to bring important projects vital to Pennsylvania’s economy.

I am taking this action now because there are fewer than thirteen months to the 2010 Pennsylvania Primary and there is much to be done in preparation for that election. Upon request, I will return campaign contributions contributed during this cycle.

While each member of the Senate caucuses with his Party, what each of us hopes to accomplish is distinct from his party affiliation. The American people do not care which Party solves the problems confronting our nation. And no Senator, no matter how loyal he is to his Party, should or would put party loyalty above his duty to the state and nation.

My change in party affiliation does not mean that I will be a party-line voter any more for the Democrats that I have been for the Republicans. Unlike Senator Jeffords’ switch which changed party control, I will not be an automatic 60th vote for cloture. For example, my position on Employees Free Choice (Card Check) will not change.

Whatever my party affiliation, I will continue to be guided by President Kennedy’s statement that sometimes Party asks too much. When it does, I will continue my independent voting and follow my conscience on what I think is best for Pennsylvania and America.

In an extensive article for the May 14th issue of the New York Review of Books, Specter writes that he intends to introduce legislation requiring the Supreme Court to review lower court decisions on challenges to Bush's warrantless wiretapping program; keeping the courts open to suits against telecommunciations companies who participated in the program; and prohibiting courts from relying on presidential signing statements.

Former Pennsylvania Senator Rick Santorum commented on Specter's decision in Politico's Arena:

I spoke with Arlen this morning and he explained his reasoning to me. I told him I was deeply disappointed that he felt he had to do it. It is a huge blow to the Republican's ability to moderate any of Obama's very liberal proposals. I can only hope that Arlen will be as independent as a Democrat as he has been as a Republican.

In his statement, Specter reaffirms his opposition to the Employee Free Choice Act: "Unlike Senator Jeffords’ switch which changed party control, I will not be an automatic 60th vote for cloture. For example, my position on Employees Free Choice (Card Check) will not change." Grover Norquist told the Washington Independent's Dave Weigel that Specter's commitment to voting against the labor bill is the "one silver lining in this decision."

Last month, Pennsylvania Gov. Ed Rendell (D) said that he, Sen. Bob Casey (D-PA), and Vice President Biden had been lobbying Specter to switch.

Assuming this comes to pass... This was likely going to be a targeted race in a very expensive state. The DSCC will have to defend, sure, but Toomey isn't electable statewide, and it should be an easy hold. That's a lot of extra money Democrats will have to target other Republican seats.

Also, this makes the GOP Senate caucus that much more conservative, and could perhaps put pressure on the Maine Republicans to consider following suit, especially Olympia Snowe who may not want to remain a forgotten member of a deep and long-lasting Southern-based minority.

Club for Growth is speechless.