Sen. Arlen Specter’s (Pa.) decision to bolt the GOP sent shockwaves through Capitol Hill on Tuesday, leaving Senate Democrats beaming and Republicans scrambling to assess the damage and chart a new path forward.

“Since my election in 1980, as part of the Reagan Big Tent, the Republican Party has moved far to the right,— Specter said in a statement. “I now find my political philosophy more in line with Democrats than Republicans.—

Specter’s decision, which was the culmination of more than five years of lobbying by Democratic leaders, including Majority Leader Harry Reid (D-Nev.), clearly caught GOP leaders off guard, who huddled in Minority Leader Mitch McConnell’s (R-Ky.) office for an hour immediately upon hearing the news.

Reid in a statement said he has had “a long dialogue— with Specter on his position in the party and welcomed his newest member into the Democratic fold. Specter, a moderate who is up for re-election in 2010, was facing a tough cycle ahead with a primary challenge from conservative former Rep. Pat Toomey (R).

“Sen. Specter and I have had a long dialogue about his place in an evolving Republican Party. We have not always agreed on every issue, but Senator Specter has shown a willingness to work in a bipartisan manner, put people over party, and do what is right for Pennsylvanians and all Americans,— Reid said, adding that, “I welcome Sen. Specter and his moderate voice to our diverse caucus, and to continuing our open and honest debate about the best way to make life better for the American people.—

Although it was unclear what if anything Reid, who met with Specter on Monday, and other leaders may have promised Specter to win his alliance, several Democratic aides familiar with the talks said it was almost certain that the Democratic Senatorial Campaign Committee would fully commit to his re-election. Additionally, several aides speculated that it is likely that the senior Senator from Pennsylvania will retain his Senate seniority and his committee assignments, although no gavels were expected to switch hands until the next Congress convenes in 2010.

Meanwhile, GOP Senate leaders were still trying to assess the fallout from Specter’s party switch. They initially tried to reject as “rumor— the news as word of Specter’s decision swept the Capitol.

“Everyone is in shock right now,— said a senior House GOP aide.

Sen. Lindsey Graham (R-S.C.) expressed disappointment in Specter’s decision, chalking it up to the fact Specter was facing a tough GOP primary next year. “Everybody switches because they’re gonna lose their primary,— Graham said, arguing that former Pennsylvania GOP Gov. Tom Ridge should be recruited to run against Specter.

Sen. Olympia Snowe (R-Maine), who has long been a close moderate ally of Specter, argued Specter’s defection proves the GOP must be more inclusive. “[Republicans] have to be inclusive. It’s more about sending your message more broadly,— Snowe said.

Specter has found himself on the side of the Democrats many times over the years, particularly on issues like stem cell research or the recently passed economic stimulus measure. He also has been a friend of conservatives, though, on issues like judicial nominations.

Democrats were clearly pleased at the news, which gives them 59 seats as they await a conclusion to the Minnesota Senate race. “Happy to have him,— Senate Finance Chairman Max Baucus (D-Mont.) said, while Budget Chairman Kent Conrad (D-N.D.) called the decision “great news.—

Sen. Frank Lautenberg (D-N.J.), who has worked with Specter on a number of regional issues, said he was pleased his longtime colleague had joined the Democratic ranks, and laid the blame for his decision at the feet of Republicans, who he said treated Specter poorly.

“In my view … he was treated disrespectfully by his party,— Lautenberg said, adding that, “I think he’s had a tough time casting some of the votes he’s been compelled to cast.—

President Barack Obama was in the Oval Office at about 10:30 a.m. receiving his daily economic briefing when he was handed a note saying that Specter was coming to the Democratic Party, according to a White House official.

Obama was able to reach Specter by phone about 10 minutes later to tell the Senator that he has his complete support and that the party “is thrilled to have you,— the official said.

Specter used to be a Democrat, but ran as a Republican when he first entered political life in 1965 and has run as a Republican ever since. In announcing his plan to return to the Democratic Party, Specter made clear he does not plan to be a “party-line voter— for the Democrats.

“Unlike Sen. [Jim] Jeffords’ switch which changed party control, I will not be an automatic 60th vote for cloture,— Specter said. “For example, my position on Employees Free Choice will not change.—

Jessica Brady, Jackie Kucinich and Shira Toeplitz contributed to this report.