Sen. George Voinovich (R-Ohio) could face more pressure than any other member of Congress next year to reverse his position on legislation that would make it easier for workers to unionize.

Voinovich voted against allowing a debate on the Employee Free Choice Act last year, and has vowed to do the same when Democrats bring it up in the 111th Congress, perhaps as early as January.

Sensing a potential swing vote, labor organizers and business groups from the Rust Belt have begun targeting Voinovich with hopes of persuading him one way or the other.

The bill, also known as card-check, passed the House last year but failed to get enough votes for a debate in the Senate. Come January, when Democrats hold at least 57 seats in the upper chamber, Voinovich’s vote could be the difference between labor’s top priority becoming law or withering away.

“If the senator is comfortable with the economy we have now, he should vote the same way he did last year,” said Tim Burga, chief of staff for the Ohio AFL-CIO, which has 1.4 million members. “If he wants a change, then he should reconsider.”

Voinovich has signaled that he won’t budge.

“It’s undemocratic,” Voinovich says of the legislation that would bar employers from insisting on a secret-ballot election if a majority of workers sign cards saying they would like to join a union. “I’ve belonged to unions. I’m probably one of the few people who understand how this works.”

Card-check is likely to be a harbinger for organized labor’s influence on a number of centrist candidates from both parties who are seeking reelection next cycle.

Already, Sen. Arlen Specter, a Pennsylvania Republican who supported moving toward a debate last year, has signaled he will seek some sort of compromise.

Democrats intend to use the card-check legislation as a means to reach middle-class voters in a slumping economy. Democratic Sen. Sherrod Brown, Voinovich’s Ohio colleague who toppled Sen. Mike DeWine (R) in 2006 with considerable help from unions, intends to make that point when the bill comes up for debate.

“The more people that belong to labor unions, the more people there are in the middle class in this country,” Brown said. “That’s been proven by statistics for 50 years.”

The pressure on Voinovich comes as the lawmaker failed to persuade his Republican colleagues to back an auto industry bailout this month that was critical to his home state and also as his electoral support seems to be softening.

After winning his 1998 and 2004 elections with comfortable margins — 56 and 64 percent, respectively — a recent poll shows he is more vulnerable in 2010 if he runs for reelection. A Quinnipiac University survey of 1,468 registered voters, conducted Dec. 4-8, found just 36 percent of respondents supported Voinovich against an unnamed Democrat. The generic Democratic candidate won 35 percent in the poll, with the remaining 29 percent of voters undecided. The poll had a margin of error of plus or minus 2.6 percent.

Voinovich, who worked closely with unions when he was the mayor of Cleveland in the 1980s, shrugs off the pressure, saying his mind is “absolutely” made up.

“To just have a situation where somebody can go out there and get X number of cards and then say, ‘By the way, we’ve got 50-plus-one, now you’re unionized’ — it’s undemocratic,” he said.

Those statements are reassuring to Ohio’s business leaders, who say they are winning the battle for Voinovich’s vote despite the pressure he faces.

“I have not heard any wavering from his office and his position,” said Tony Fiore, director of labor and human resources policy for the Ohio Chamber of Commerce. “And it is important for all of those folks who have been on the right side of this to continue their strong opposition.”

The U.S. Chamber of Commerce is applying pressure at the national level. Chamber Vice President Randel Johnson calls the bill “the top labor issue of the last 50 years” and said if Democrats get the 60 votes needed to proceed to a debate, the bill is all but certain to become law.

Senate Minority Leader Mitch McConnell (R-Ky.) not only said that Republicans would unite against the legislation, but predicted Democrats may peel away from the bill.

“You are likely to have very significant unity among Republicans,” McConnell said. “Also, you’re assuming that Democrats are going to walk in lockstep. I would predict there are going to be a lot of differences among Democrats in both the House and the Senate.”