WASHINGTON -- Veteran Pennsylvania Sen. Arlen Specter said Tuesday he will become a Democrat, a sudden shift that presents Sen. Harry Reid of Nevada with an opportunity to consolidate his power as majority leader with more muscle than ever to get bills passed for President Barack Obama.

The switch by the five-term Republican puts Reid a step closer to claiming 60 votes in the Democratic caucus, the magic number that would allow him to overcome GOP filibusters that thwart Obama initiatives. Specter becomes No. 59, with the goal of 60 achievable if courts confirm humorist Al Franken's possible 312-vote victory in Minnesota.

"If this is not a tipping point, it is really close," said Eric Herzik, a professor at the University of Nevada, Reno. "Republicans know that Reid just got stronger."

Specter, 79, conceded bluntly that his chances of winning a Republican primary in Pennsylvania next year were bleak in a party grown increasingly conservative. But he cast his decision as one of principle, rather than fueled by political ambition as spurned GOP leaders alleged.

"I have found myself increasingly at odds with the Republican philosophy and more in line with the philosophy of the Democratic Party," he said at a news conference.

In the same motion, the remaining Senate Republicans took a step deeper into the wilderness, their ability to influence legislation greatly diminished. If Franken is seated, there will be only 40 Republicans, their smallest number since 1979.

"It is a disappointing day for all Republicans," said Sen. John Ensign, R-Nev. He said GOP senators have to hope that Reid will be open to their ideas, or else Democrats will "cram down their agenda."

"So far it has not been bipartisan, and we know that," Ensign said.

Other Republican reaction was mixed. Sen. Olympia Snowe of Maine called the defection "devastating news" and a wake-up call for the GOP to treat its moderate members better or face becoming a marginalized, mostly Southern party.

Sen. John Cornyn of Texas, chairman of the National Republican Senatorial Committee, dubbed Specter's decision "the height of political self-preservation."

Specter could provide key votes to move Obama's health care reform, said Danny Gonzales, a political science professor at Great Basin College in Elko. Others said the longtime member of the Senate Judiciary Committee could help the president confirm moderate judges.

Less certain is how the switch would affect the proposed Employee Free Choice Act, the card check bill, which is sought by unions to help them organize in workplaces. Specter said he intends to stick by his decision, announced several weeks ago, to oppose that bill in its current form.

Mark Peplowski, who teaches at the College of Southern Nevada, predicted Reid will show restraint with his new tool.

"He will remind his membership that just because they have 60 votes, they cannot go hog wild because the public expects them to get things done," Peplowski said.

In typical Reid fashion, the recruitment was done largely in secret. In that way it was similar to Reid's courtship of the late Sen. James Jeffords, who was persuaded to leave the Republican Party and caucus with Democrats as an independent in 2001.

"One reason I have had a bit of success in encouraging people to come to our caucus is by keeping it very close to my vest," Reid told reporters Tuesday. "There isn't a single member of my caucus that knew how far along I had gotten with Senator Specter."

Reid and Democrats had courted Specter on and off for several years. Reid said the conversations "became more intense in the last month or two."

Vice President Joe Biden has talked to Specter almost weekly and has met with him six times since mid-February, said a White House aide who spoke on the condition of anonymity because she was not authorized to disclose the information.

Specter was one of three Republicans who voted for Obama's $787 billion economic stimulus package.

He has been among a shrinking number of moderates in the Republican Party, which has come to be dominated by conservatives. Specter said his decision was made after learning from his pollster on Friday that he probably would not win re-election to a sixth term next year as a Republican or an independent.

Obama and Reid promised to campaign for him if he became a Democrat, and Reid promised he would not lose seniority in the Senate. That means Specter could become the No. 2 Democrat on the Judiciary Committee and fourth-ranking on the Appropriations Committee.

Specter told Reid of his decision about 6 p.m. Monday, then said goodbye a short time later to Sen. Mitch McConnell, the Republican leader from Kentucky.

Specter called the White House on Tuesday to notify Obama of his decision to switch. The president called back moments later, spokesman Robert Gibbs said, to say the Democratic Party was "thrilled to have you."

Not long after Specter met privately with Republican senators to explain his decision, McConnell said the switch posed a "threat to the country." The issue, he said, "really relates to ... whether or not in the United States of America our people want the majority party to have whatever it wants, without restraint, without a check or balance," McConnell said.

Specter gets the Democrats closer to a filibuster-proof majority on paper, but he warned that he will not be a rubber stamp, which could complicate Reid's task as he tries to hold together a caucus that includes liberals such as Sen. John Kerry of Massachusetts and conservatives like Sen. Ben Nelson of Nebraska.

"I will not be an automatic 60th vote for cloture," Specter said. "My change in party affiliation does not mean that I will be a party-line voter any more for the Democrats than I have been for the Republicans."

"Certainly I do not count him as an automatic vote," Reid said. "I don't count anyone in my caucus as an automatic vote, but I do respect his willingness to work in a bipartisan manner."

Gonzales predicted Republicans now will go to the mat for their candidate Norm Coleman in Minnesota as he challenges Franken in court.

"I don't see Republicans having any other choice right now," Gonzales said. "They are going to have to use every resource they have. I see the Republicans getting aggressive on this front."

The Associated Press and McClatchy Newspapers contributed to this report. Contact Stephens Washington Bureau Chief Steve Tetreault at stetreault@stephensmedia.com or 202-783-1760.