At a reception in late February honoring Sen. Arlen Specter for his death-defying vote that secured passage of the $787 billion stimulus bill, Gov. Rendell joked that his old friend should just make it easier on himself by changing his registration from Republican to Democrat.

"In his heart, he knows he's a Democrat," Vice President Biden added when it was his turn to dish out the praise that day in an Irvine Auditorium recital room at the University of Pennsylvania.

Specter demurred that he would stay put because it was important to have moderates in the GOP.

But even as he was speaking, Specter's standing among Republican primary voters, often shaky, was eroding fast.

Last week, he jumped.

When he announced Tuesday that he was turning Democrat, Specter cited polling that showed his approval rating plummeting by half among Pennsylvania Republicans, with the party's increasingly conservative base inflamed by his stimulus vote.

The numbers convinced the five-term senator he could not win the 2010 primary against conservative former Rep. Pat Toomey, who had headed the Club for Growth, which targets moderate Republicans it considers soft on fiscal issues.

At the same time, top Democrats had engaged in a behind-the-scenes effort to persuade Specter to switch parties - especially Biden, a close friend from their Senate days. For years, the two men rode the Amtrak train home from Washington together, Biden to Wilmington and Specter to Philadelphia.

"I believe and I kidded him all along since January that he was going to switch," Biden said in an interview. "He said, 'No, I'm not going to switch from the Republican Party. I can change their minds.' It was clear to me that he was so principled and independent . . . that there would come a point where he hit a wall."

Specter, in an interview, confirmed that Rendell and Biden in recent weeks had intensified their efforts to persuade him to become a Democrat.

"Biden has been persistent," Specter said. Indeed, White House officials said the vice president had 14 telephone conversations or meetings with Specter since the stimulus vote Feb. 13.

"I didn't count 'em, but I have seen a lot of him just in the general course of business," Specter said of their contacts. "He didn't talk to me about changing all the time, but Biden has pushed it consistently."

There was no "hard sell," Specter said, adding that everybody knew such an approach would be counterproductive with him.

Biden agreed: "No one induces Arlen," though he said their conversations are "unfettered."

Biden said he started making the case for becoming a Democrat just before Specter's 2004 primary against Toomey, who would come within 17,000 votes of unseating him.

"I think for some time on the really big-ticket items relating to economics and social policy, safety-net issues, labor - a whole range of things - Arlen's been basically a moderate Democrat," Biden said. Until last week, Biden said, Specter thought he had room to broaden the GOP coalition "to build a big tent."

Specter, 79, said his decision to switch was sealed after final survey results from his own campaign pollster, Glen Bolger, came in April 24.

"The most important number was the approval rating - it dropped from the 60s to 31" percent just in the last few months, Specter said.

Not that long ago, Specter drew standing ovations from mostly conservative crowds around the state as he stumped with Republican presidential candidate John McCain.

But the stimulus vote was a "watershed," Specter said. "It all turned on that. The pollsters had never seen that kind of precipitous drop. It was stark."

The survey found Specter trailing Toomey among Republicans by 15 percentage points in a three-way matchup with antiabortion candidate Peg Luksik, according to sources familiar with the findings. More important, a large majority of those listed as undecided described themselves as "conservative" or "very conservative," meaning that "the pool was such we couldn't overcome" the deficit, one source said.

"The numbers reflected the exodus of moderates from the party in the eastern part of the state," said the source, who requested anonymity because he was not authorized to disclose the results.

A Quinnipiac University poll released March 25 also painted a stark picture: Only 29 percent of Pennsylvania Republicans surveyed approved of Specter, while 60 percent of Democrats and 41 percent of independents looked on him favorably.

An unpublished poll of state Republican voters by Franklin and Marshall College - which was being tabulated Tuesday, as Specter announced his decision - found that only 29 percent believed he deserved reelection, with 60 percent agreeing it was "time for a change."

Of those Republicans who wanted Specter out, 25 percent said they disagreed with him ideologically, 11 percent said he was too supportive of President Obama, and 20 percent said he had been in office too long. Eight percent cited concerns about his age or health.

"There is just no doubt he made the right choice politically," said Berwood Yost, director of the Center for Opinion Research at F&M. "He was in a difficult spot."

The poll was based on telephone interviews conducted April 21 through Monday with 374 registered Republicans in Pennsylvania and has a margin of error of 5 percentage points.

Specter's decision to leave the party he joined in 1966 was made after dinner Sunday night at the home of his older son, Shanin, outside Philadelphia. Specter said he'd had time to read over and reflect on the campaign poll and several recent public polls, and he discussed his options with his wife, Joan; his son; and his granddaughter Silvi, 15.

His wife and son were more definite about the need to leave the GOP, Specter said. "My hesitation was on . . . giving up the fight. I don't like to give up the fight."

Just before the kitchen-table meeting, Specter attended a fund-raiser at the home of lawyer Mark Aronchick with about 50 Democrats who wanted to help him defeat Toomey. As Specter was discussing the difficult primary ahead, Aronchick recalled, one guest asked why the senator didn't just become a Democrat.

"He said, 'Well, we'll see how things develop,' " Aronchick said. "I listened to that and thought, 'Whoa.' "

Specter kept the decision close. He informed Senate leaders Monday night, then called Biden, Rendell, and other leaders Tuesday morning just before making his bombshell announcement.

"When people say I have a personal interest, of course I do - but my 30 years of seniority doesn't belong to me, it belongs to the people of Pennsylvania," Specter said. "I boil it down to: I changed parties so I could stay in the Senate and use my senior position" to help the state.

"What people aren't understanding is, he's not ideological at all," said Roger Stone, a GOP political consultant who has advised Specter for years. "He's a policeman, a cop. He sees things as right or wrong, not liberal or conservative. . . . He wants all the evidence before he makes a decision."

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