In the wake of moderate Sen. Arlen Specter’s (R-Pa.) decision this week to oppose one of their top priorities, Senate Democrats already are trolling for another Republican swing vote to help them advance controversial legislation this Congress.

A Senate Democratic leadership aide said Majority Leader Harry Reid (D-Nev.) was taken aback by Specter’s unexpected announcement on Tuesday that he would oppose a Democratic-backed measure to make it easier for employees to join unions. Specter is facing a likely primary challenger in 2010 and has come under fire at home for being one of three Republicans who last month supported a $787 billion economic stimulus bill.

Specter had been seen as critical to Senate passage of the pro-labor “card check— bill this year. Democrats say his pre-emptive strike — announcing he would oppose moving it through the chamber — signals that his days as an ally are over, at least for the next two years.

Specter, in his floor speech on the labor bill, specifically addressed the pressures of being the “decisive— vote on a controversial bill in a closely divided Senate. The Democrats control the chamber 58-41, with one seat — Minnesota — still unresolved.

“In a highly polarized Senate, many decisive votes are left to a small group who are willing to listen, reject ideological dogmatism, disagree with the party line and make an independent judgment,— Specter said. “It is an anguishing position, but we play the cards we are dealt.—

Specter has already felt significant political blowback for helping pass the stimulus bill, which was opposed by most Senate Republicans and was a rallying point for party conservatives. A recent Quinnipiac University Polling Institute survey showed that Specter’s likely primary opponent, conservative former Rep. Pat Toomey (R), is leading Specter by a comfortable margin, which Specter himself acknowledged is a result of his support for the stimulus bill.

“My role in it arose because I provided a critical vote getting to 60. That’s why it happened,— Specter said on Wednesday, noting that “at one time we had six people there … and like Indians, they dropped off, one by one.—

“So, the real role I had was providing a critical vote. And I understand that. And is that the reason why the Quinnipiac poll is a big problem for me? I think so.—

Sen. Susan Collins (Maine), another key GOP moderate who also backed the stimulus, said that while it can be difficult to be on the opposite side of her leadership, especially when that vote is portrayed as the decisive one in passing a bill, it is a reality that comes with being a Senator. “It does at times create some stress, but that’s part of the job,— Collins said.

“I’ve done that several times myself already— this year, she said.

Like Specter, Collins is one of the handful of lawmakers Reid has called on to try to push through controversial measures. Collins said she will always support “my party when I can, and reluctantly oppose it when I must.—

But unlike Specter, who is facing a tough primary race from the conservative wing of his party, Collins has rarely had to deal with opposition from a fellow Republican. Last year, she easily beat back a challenge from then-Rep. Tom Allen (D) in a state where an independent streak is welcome.

In Specter’s case, Toomey has not officially announced he is running for Senate, but he has indicated that he is likely to do so. Specter defeated Toomey by a very slim margin — fewer than 17,000 votes — in the 2004 Republican primary.

If the recent Quinnipiac poll is accurate, Specter could be in even hotter water for the 2010 Republican primary. The survey results showed Specter trailing Toomey, 41 percent to 27 percent, with 28 percent undecided.

Specter trailed Toomey despite being much better-known in the state. Specter received a 45 percent favorable rating from those surveyed — down from 55 percent in a similar survey in August. Toomey had a 14 percent favorable rating, but 78 percent of respondents said they were not familiar with the former Congressman.

What’s more, numbers from the Pennsylvania secretary of state’s office show that many Republicans have switched their registration to Democrat over the past six years, creating a more conservative, closed GOP primary this time around.

But even if he wins his party’s nod in the primary, Specter could have even bigger problems down the road in the general election. The poll also showed Specter in a statistical dead heat with a general Democratic primary opponent, 33 percent to 31 percent.

Democrats, however, have yet to put up a top-tier candidate to run against him. Former National Constitution Center President Joe Torsella has announced his candidacy, but several sitting Members — including Reps. Joe Sestak, Patrick Murphy and Allyson Schwartz — also are considering a bid.

Another complication: Unions remain a powerful force in Democratic politics in Pennsylvania, and they could be emboldened to campaign against Specter given his recent switch on the pro-labor legislation. Before his announcement, union leaders had indicated that they would consider supporting Specter for re-election if he supported the bill.

Collins said that while party loyalty is important, “in the end you have to do what you think is right for your constituents,— a process that in many ways is easier because “for my part, I represent a state that prizes independence.—

Indeed, while Maine is in many ways fairly monolithic in its demographics and political leanings, Pennsylvania is much more varied. The heavily urban areas trend Democratic, but in the rural areas, Christian conservative values hold sway.

David M. Drucker contributed to this story.