Pressed by labor officials and leaders of his new party, embattled Sen. Arlen Specter is one of the players in multiparty negotiations seeking a compromise bill that would make it easier for unions to organize new workplaces.

The former Republican expressed optimism yesterday that an accord could be reached on the legislation, and a senior labor official cited progress toward agreement.

Uncertainty remains, however, on whether and when deals can be struck to overcome stumbling blocks on election procedures and rules to ensure that employers and new unions reach contracts within a reasonable period of time.

The measure, the Employee Free Choice Act, is a key priority of the labor movement in this Congress. Mr. Specter was its only Republican supporter on a crucial procedural vote on similar, unsuccessful legislation two years ago -- a vote that supporters of the bill have reminded Pennsylvanians of in a prominent round of television commercials. But in one of his last acts before leaving the GOP, Mr. Specter announced earlier this year that he would vote against the bill in its current form. Mr. Specter said he favored labor law reform in general, but said he was particularly opposed to a provision that would permit unions to be recognized without a requirement for a secret ballot election. He also faulted the bill's call for binding arbitration in cases where unions and employers failed to negotiate an initial contract.

In an interview with The Associated Press yesterday, Mr. Specter said of the ongoing talks, "I think the prospects are pretty good. It's impossible to predict with certainty, but I think the prospects are pretty good."

Mr. Specter, according to a transcript of the exchange provided by his office, was circumspect about the details of a prospective agreement.

"That does not lend itself to a short answer. And it's not advisable to start talking about compromises," he said. "That's something for negotiations."

Bill Samuel, director of government affairs for the AFL-CIO, said the labor group has had repeated contacts with Mr. Specter on the bill since his statement of opposition.

"We're making progress," he said. "He's putting a lot of energy into it. ... I think he is genuinely seeking a bill that would pass the Senate and achieve our goals."

There are multiple players in the talks. After Mr. Specter declared his opposition, several centrist Democratic senators expressed similar reservations. Another Democrat, Sen. Dianne Feinstein of California, has also criticized the bill's election procedures, a lightning rod for its business opponents.

Under the existing proposal, employers would not be permitted to demand a secret ballot election if a majority of workers signed cards supporting a union. The bill's opponents portray that approach as a potential enabler of coercion by union organizers. Its supporters argue, in contrast, that this so-called "card check" feature is needed to circumvent coercion by union opponents.

Ms. Feinstein has suggested a compromise whereby organizing elections could be conducted by mail. If a majority mailed pro-union cards to the National Labor Relations Board, the union would automatically be recognized. Sen. Tom Harkin, a key supporter of the bill, has reportedly welcomed the Feinstein suggestion.

But Mr. Harkin has also been a strong supporter of the bill's call for mandatory arbitration in cases where talks do not produce a contract, a provision that Mr. Specter singled out for criticism in his recent rejection of the bill.

The political stakes for Mr. Specter are clear. While his political conversion brought the embrace of senior Democrats, including President Barack Obama and Gov. Ed Rendell, labor leaders have done their best to hold his feet to the fire on their long-sought organizing legislation. While the state AFL-CIO and other labor groups have supported Mr. Specter against Democratic opponents in the past, they have made clear that their support next year could turn, depending on where Mr. Specter stands on the organizing legislation.

Their leverage is enhanced by the possibility of credible opposition to Mr. Specter in next year's Democratic primary. U.S. Rep. Joe Sestak, D- Delaware County, has stopped just short of declaring his candidacy against the five-term veteran, although sources close to Joe Torsella, the first declared Democratic hopeful for the seat, confirmed that he is dropping his short-lived bid. That still leaves Michael Lamb, the Pittsburgh city controller, and state Rep. Bill Kortz, of West Mifflin, who are also eyeing possible challenges to Mr. Specter.

The campaign of former Rep. Pat Toomey, the Republican who effectively drove Mr. Specter out of the GOP and its 2010 primary, greeted the reports of Mr. Specter's negotiations with a "Specter Flip-Flop Alert,'' e-mailed to reporters across the state.

"Arlen Specter had the right position on card check for a grand total of six weeks,'' said Toomey aide Nachama Soloveichik. "But it is clear his short-lived support for workers was motivated by politics, not principle. ... This kind of power-hungry flip-flopping is exactly the kind of behavior that upsets Pennsylvanians regardless of their party affiliation.''