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WASHINGTON (CNN) -- Veteran Pennsylvania Sen. Arlen Specter said Tuesday that he switched from the Republican to the Democratic Party.

"I know I'm disappointing many of my colleagues," he said at a news conference. "The disappointment runs in both directions.

"I'm putting principle at the top of the list," he added.

When asked how and when he made the decision, Specter said, "the decision has been reached as I have traveled the state [Pennsylvania] in the last several months. Specifically, I got my home poll results last Friday ... and consulted with my campaign managers and had a long discussion. ... I came to a decision over this past weekend."

Specter also said that President Obama and Senate Majority Leader Harry Reid would campaign for him as he runs for re-election -- now on the Democratic ticket -- in the 2010 race.

The Specter party switch would give Democrats a filibuster-proof Senate majority of 60 seats if Al Franken holds his current lead in the disputed Minnesota Senate race.

"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 posted by his office on PoliticsPA.com.

"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."

Specter, a five-term Senate veteran, was greeted by a loud, sustained round of applause by dozens of constituents outside his Washington office shortly after the news broke.

"I don't have to say anything to them," a smiling Specter said. "They've said it to me."

Obama called Specter shortly after learning the news during his daily economic briefing in the Oval Office on Tuesday morning, according to a senior administration official.

"You have my full support, and we're thrilled to have you," Obama told Specter.

Jubilant Senate Democrats also welcomed the news.

"Sen. Specter and I have had a long dialogue about his place in an evolving Republican party," Reid, D-Nevada, said in a written statement.

"We have not always agreed on every issue, but [he] 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 called Specter a "man of honor and integrity" who would be welcome in the Democratic caucus.

One key Senate Democrat warned that reaching the 60-vote mark would not automatically ensure a Democratic victory on every major issue.

"It's great news," North Dakota Sen. Kent Conrad said. But it means "a lot less" than some people think.

"The Democratic caucus is not homogenous. [There is a] lot of disagreement in the Democratic caucus, so this idea that it's some great watershed event ... I don't think so."

Republican National Committee Chairman Michael Steele ripped Specter, calling him a Republican in name only who was out of step with the rest of the party because of his "left-wing voting record."

"Some in the Republican Party are happy about this. I am not," Steele said in a written statement. "Let's be honest --Sen. Specter didn't leave the GOP based on principles of any kind. He left to further his personal political interests because he knew that he was going to lose a Republican primary due to his left-wing voting record."

Steele said Republicans "look forward to beating Sen. Specter in 2010, assuming the Democrats don't do it first."

Specter was expected to face a very tough primary challenge in 2010 from former Rep. Pat Toomey, who nearly defeated Specter in the Pennsylvania GOP Senate primary in 2004.

A Quinnipiac University survey of registered Pennsylvania voters released last month showed Specter trailing the more conservative Toomey in a hypothetical primary matchup, 41 to 27 percent.

A separate Franklin & Marshall survey showed Specter leading Toomey 33 to 18 percent. Another 42 percent, however, were undecided.

More than half of the Republicans polled in the Franklin & Marshall survey said they would prefer to see someone new in the Senate.

Numerous Republicans are angry with Specter over his recent vote in support of President Obama's $787 billion stimulus plan.

Specter, one of only three GOP senators to vote for the measure, has been part of a dwindling group of GOP moderates from the northeastern part of the country.

The 79-year-old former Philadelphia district attorney won his first of five Senate terms in 1980. He has been a leading Republican on the Senate Judiciary Committee for much of the past two decades, serving as its chairman from 2005 to 2007.

Specter served on the Warren Commission, which investigated the 1963 assassination of President Kennedy. He has survived bouts with cancer three times, most recently undergoing chemotherapy for Hodgkin's disease in 2005.