Democrats want Sen. Arlen Specter (R-Pa.) in their ranks - and they've been sending in their big guns to lobby him to make the switch.

    Pennsylvania Gov. Ed Rendell (D) said he, Vice President Biden and Sen. Bob Casey Jr. (D-Pa.) have tried to get the longtime senator and centrist Republican to come to their side of aisle but that so far he won't budge.

    Specter, who faces a tough GOP primary in his reelection bid, started his career as a Democrat. He was a prominent Philadelphia lawyer active in Democratic politics before he switched sides and became the city's district attorney on the Republican ticket.

    Rendell told a Northeast cable station that he, Biden and Casey have all tried to convince Specter to switch parties.

    In the interview with the Regional News Network, Rendell also said Specter would be a shoo-in as a Democrat.

    "We've tried - myself, Sen. Casey, Vice President Biden," Rendell said. "We've tried to talk him into it, but he's bound and determined to stay a Republican. He doesn't want to see Republican moderates vanish from the earth."

    In response, Specter's office referred to a recent radio interview in which Specter said he would run with the GOP.

    "I'm prepared to say that I'm running as a Republican," he told conservative host Michael Smerconish.

    Rendell said the welcome would be warm for Specter, who has spurned his party on the recent stimulus package and could do so again on the Employee Free Choice Act, also known as "card-check."

    "The Democrats in the Senate would welcome him," Rendell told the station. "I think he'd be basically unopposed for the Democratic nomination, and I think he'd win 60, 65 percent of the vote in the general election."

    Democrats have no strong front-runner for Specter's seat at this point in the 2010 cycle. If the senator switched parties during the 111th Congress, he could be the 60th vote for Democrats in the Senate, a number needed to end debate on legislation.

    Biden's office declined to comment Monday, and Casey's office said Casey has never lobbied Specter privately.

    Rendell's office on Monday confirmed the governor's comments, but said that Rendell might have been referring to more casual lobbying efforts by other Democrats.

    "The governor has had conversations - both public and private - trying to urge Sen. Specter to switch parties, and each and every time, he has received a resounding 'no' in response," Rendell spokesman Chuck Ardo said.

    Ardo agreed with Casey's office that Rendell might have been referring to an event last month at which the governor and Biden joked publicly with Specter about switching parties.

    During the event, which was held soon after Specter's vote for the stimulus, Rendell chided the senator for being a Democrat in Republican's clothing.

    "You could make life a little easier for yourself by taking that registration card of yours and making that little change from 'R' to 'D,'" Rendell said to Specter, according to a pool report from the event.

    The report also said Biden joked that Specter knew in his heart that he was a Democrat.

    Specter looks to be in serious trouble in the GOP primary, with former Rep. Pat Toomey set to run against him for the second straight election. Toomey lost by less than 2 percent in 2004, and Specter is polling horribly with the GOP base.

    A Susquehanna poll from late February showed 66 percent of Republicans favoring a new senator and just 26 percent supporting Specter.

    Since it was reported two weeks ago that Toomey will again run against Specter, speculation has been that Specter might need to switch parties or run as an Independent if he wants to have a chance at getting reelected.

    A sly Toomey said Monday that he sympathized with the Democratic effort.

    "Given Sen. Specter's voting record and the interest of senior Democrats in wooing him, it's pretty clear that, substantively, he is a Democrat," Toomey told The Hill. "I wish he would switch to the Republican Party."

    For now, the rumor mill in Pennsylvania will continue to roll.

    The state has closed primaries, meaning independents can't vote for Specter in the party nominating contest.

    Complicating matters for Specter is the fact that his party is hemorrhaging voters.

    Republican registration has remained stagnant since Specter's 2004 primary, while Democratic registration has risen by about 750,000 and other affiliations have climbed nearly 200,000 - shifts that suggest a broadening of the Democratic pool and a more hard-right GOP electorate.

    There has been some talk that Specter or his supporters could try to get state law changed to allow for an open primary, or that labor groups would convince their followers to register as Republicans to vote for him, so long as he votes in favor of the Employee Free Choice Act.

    Republican National Committee (RNC) Chairman Michael Steele has said on multiple occasions that the national party might withhold funding from candidates who spurn the party on major issues - a clear threat to Specter and his fellow stimulus defectors, GOP Maine Sens. Susan Collins and Olympia Snowe.

    Pennsylvania law prohibits Specter from running as an Independent if he loses the GOP nomination, as Sen. Joe Lieberman did following his loss in the 2006 Democratic primary in Connecticut.