Updated: 5:24 p.m.

Legislation aimed at making it easier for employees to join unions was dealt what may be a fatal blow Tuesday when Sen. Arlen Specter (R-Pa.) announced he would filibuster the bill.

Specter had been the only Republican to previously vote for cloture on so-called “card check— legislation, which would allow a majority of workers at a company to create a union by signing union cards. The legislation also would create a binding arbitration process once a union is formed to prevent employers from bargaining in bad faith.

Specter opposed both central tenets of the legislation in a speech on the floor of the Senate Tuesday afternoon.

Specter voted for cloture to proceed on the card check bill in 2007 when Republicans had a stronger minority hand in the Senate with 49 seats. Now, Democrats have 58 seats to the GOP’s 41, meaning that without Specter’s support Democrats have little chance of getting to the 60 votes they need to cut off debate and move forward on the bill.

The moderate Pennsylvania Republican called his decision a “close call— and said he might consider changing his mind when the economy improves if the bargaining position of unions has not improved.

He acknowledged that he was likely the deciding vote on the issue and said he had spent a lot of time deliberating.

“It is very hard to disappoint many friends who have supported me over the years, on either side, who are urging me to vote their way,— he said. But Specter, who is up for re-election in 2010 and likely to face a tough primary challenge from the right flank of his party, said his decision was not motivated by political advantage.

Specter said that the loss of the secret ballot was at the top of the list of his concerns on the bill, but he also worried that binding arbitration could subject employers to deals they cannot live with.

Specter instead suggested a list of changes to existing labor laws that would enhance the positions of unions, although to a much more limited effect. He proposed imposing tighter timetables on union elections, new rules aimed at leveling the playing field for campaigning, and new penalties for violating labor laws.

The veteran Senator’s position was immediately applauded by Republican-friendly business groups like the U.S. Chamber of Commerce, which has engaged in an all-out battle with unions over the issue. Card check is the labor unions’ top priority for the Congress and one of business interests’ biggest concerns.

“There will likely be many attempts to push other forms of labor law reform with similar goals, and we urge the Senator to continue to oppose these misguided efforts,— said Thomas J. Donohue, president and CEO of the chamber.

As business groups cheered over Specter’s announcement, labor interests expressed disappointment. American Rights at Work Executive Director Mary Beth Maxwell, calling the card check legislation “critical” to restoring the middle class, said Specter’s decision is “inconsistent with his own record of support for working people.”

Specter’s decision on the card check issue might come too late to prevent a primary challenge from former Rep. Pat Toomey (R-Pa.), who lost to the Senator in the 2004 GOP primary by less than 3 percent and has indicated that he will likely challenge him again in 2010. In a statement, Toomey attributed Specter’s change of heart to the politics surrounding his re-election.

“The difference between Specter’s vote on the big government stimulus bill and Specter’s vote on card check: a threat in the Republican primary,— Toomey said. “It’s nice to see Sen. Specter reverse his position in a positive direction on card check, but I wish it didn’t take primary opposition to get him to do it.—

Toomey, who also heads the fiscally conservative Club for Growth, also released a second statement on Specter leaving the door open to reconsider the bill if the economy rebounds — or as Toomey put it, “if he thinks his political fortunes have improved, he will deny workers a secret ballot after all.—

Specter’s turnaround will likely still hurt him in the closed GOP primary, which has become more conservative over the past six years as many Pennsylvania Republicans switched their registrations. Unless Specter can lead a registration effort to bring in those voters as Republicans — an effort that unions had reportedly offered to help with if Specter supported the card check bill — he will be looking at a difficult path to re-election in 2010.

What’s more, Specter’s decision also puts him in hot water for the general election. Labor unions remain a powerful force in Keystone State elections and will now likely support the Democratic nominee voraciously.

National Democrats have indicated that they plan to target the Pennsylvania Senate seat, whether or not Specter wins his primary. Former National Constitution Center President Joe Torsella is the only announced candidate; however, several sitting Members are also considering a bid.