The business-feared, loved-by-unions Employee Free Choice Act (EFCA) was introduced in both the House and the Senate on Tuesday. Passage by the House seems a done deal, but in the Senate success hinges on getting 60 votes to choke off a filibuster.

With 58 (and potentially 59 with Al Franken) Democratic Senators, invoking cloture wouldn’t seem like such a high hurdle, but a handful of Senate Dems is having reservations about EFCA while Senator Arlen Specter of Pennsylvania, who voted for EFCA the last time around, is more hesitant in the midst of a re-election tussle.

Also, consider this: The Senate bill listed 40 co-sponsors, six fewer than in 2007. The House bill had 223 co-sponsors, compared with 230 co-signers on an earlier version of the legislation.

Already, there’s talk that the proposed law might be modified, and some have even speculated on a “grand bargain” in which the card-check provision is stripped out but the other portions are left intact.

Card check is what opponents like to call the EFCA since the law would allow workers to unionize once 50 percent plus one of them sign an authorizing card. A secret ballot would no longer be required but would be an option–at the workers’ choosing, not the employers’.

Even with card check out, the EFCA would still create many headaches for businesses. Its other provisions increase penalties for interfering with union organizing efforts and also mandate binding arbitration if union and company can’t agree on a contract. In fact, the final result of the arbitration could be a dictated contract forced down the company’s throat!

Representative Joe Sestak (D.-PA) has already introduced an alternative piece of labor legislation–the National Labor Relations Modernization Act–that basically removes card check but leaves the other provisions in.

Senator Specter, who voted with the Democrats to end the filibuster when the EPCA was first introduced in 2007, is no longer a sure “yes” vote. He’s facing an extremely tough primary opponent, Representative Pat Toomey, who vociferously opposes EFCA and almost beat him last time around. Plus, Specter himself authored a paper last summer for a Harvard symposium, in which he recounted horror stories of union lies and intimidation employed to trick and force workers into signing authorization cards.

At any rate, as Personnel Concepts has reported in its white paper on labor law under Obama, the battle over EFCA should be grand drama.