Vice President Biden said Wednesday he believes Pennsylvania Sen. Arlen Specter would have "an open mind" on voting for a bill that makes it easier to form unions if a compromise emerges.

Specter's party switch puts Democrats within reach of a 60-seat majority, which could overcome an expected GOP filibuster of the controversial Employee Free Choice Act.

The Pennsylvania senator disappointed labor leaders last month when he said he would not support the bill -- also known as card check -- in its current form. But he left the door open to supporting other labor reforms.

Biden told regional reporters in a telephone conference that he had not spoken with Specter about his position on the bill.

"But knowing Arlen, I believe Arlen will probably have an open mind if there's a compromise offered," Biden said. "But I don't know. That's my 34-year history of dealing with him. He's an intelligent, open-minded guy and I think he would listen to alternatives."

Biden said he remains hopeful that Congress will pass the bill, saying the White House is in "constant discussion" about it with labor and business groups.

"We are working very closely with labor, we support card check," Biden said.

Business groups have spent millions lobbying against the bill, which would take away the right of employers to demand secret ballot elections if workers indicate they want to form a union. Instead, the bill would give employees the right to organize by signing cards. The measure also calls for binding arbitration if management and the union cannot agree on a first contract.

Specter reiterated his opposition to those provisions when he announced his party switch on Tuesday.

"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," Specter said. "For example, my position on Employees Free Choice (Card Check) will not change."

Still, Specter's comments last month that he would reconsider if changes were made to the bill has union leaders giddy about his party switch. They sense he will be free to soften his stance on the measure as Democrats work out a compromise.

"It's a huge change for labor and for progressives everywhere," said AFL-CIO legislative director Bill Samuel. "This provides the room, it provides space for him to continue this discussion and pass a meaningful bill."