As the Democratic presidential primary spotlight shifts to Pennsylvania, unions, Members of Congress and free trade- agreement critics are stepping up their efforts to ensure that the issue will take center stage in the Keystone State as it did in this week’s Ohio contest.

Both Sens. Hillary Rodham Clinton (D-N.Y.) and Barack Obama (D-Ill.) blasted the 1993 North American Free Trade Agreement and other trade pacts, saying the measures have cost the United States jobs and hurt the economy.

This renewed focus on NAFTA is a victory for trade critics, but it has put the Bush administration in the position of defending long-passed agreements while trying to resuscitate its beleaguered trade agenda on Capitol Hill this year.

“I’m very pleased to see that trade has definitely been a focal point in some of the [primary] states like Ohio and, actually, Texas,” said Rep. Mike Michaud (D-Maine), a co-founder of the House trade working group, which opposes pending agreements with Colombia, Panama and South Korea. “It’s definitely going to be a major issue in Pennsylvania, North Carolina and West Virginia, so I’m glad the presidential candidates on the Democratic side are talking about trade.”

Michaud said he’d like to hear more specifics from Clinton and Obama and is working to organize conference calls with both candidates and the 25 other members of the trade working group — many of whom, like Michaud, are uncommitted superdelegates.

U.S. Trade Representative Susan Schwab, who will testify today before the Senate Finance Committee, called the NAFTA talk “unproductive.” The trade agreements, she said at an Institute for Education policy briefing on Wednesday, are a “plus” for the U.S. economy and a solution for, not a cause of, the nation’s economic ills.

And she stressed that trade liberalization proponents must tell their story, as deals with Colombia and other nations hang in the balance. This week, her office released a two-page report, “NAFTA — Myths vs. Facts,” that said U.S. employment rose between 1993 and 2007 by 24 percent and unemployment declined.

But on a conference call with reporters, Leo Gerard, who heads the Pittsburgh-based United Steelworkers union, called those statistics “bullshit.” His union, which endorsed John Edwards, is holding back its endorsement for now and is urging the two remaining candidates to put trade at the forefront, especially in Pennsylvania, which will vote on April 22.

“I appreciate that the leading Democratic candidates had a spirited discussion about NAFTA, but the fact of the matter is you can’t fix NAFTA” by simply adding environmental and labor provisions, he said. “We need to sit down and negotiate within the House and Senate a new set of trade rules that will create jobs in America.”

Gerard, who opposes the other pending deals, also said his union and other allies would work to put trade on the top of the issues in the general election in November. He added that he thinks Sen. John McCain (Ariz.), the Republican nominee, has gotten a “free ride” and should be criticized for supporting European company Airbus in a Department of Defense contract for an in-air refueling plane.

On the same phone call, Sen. Sherrod Brown (D-Ohio), a vocal trade critic, said that both Clinton and Obama offer a different trade policy, but McCain is essentially “running for a third Bush term.”

Business groups are, as Schwab has suggested, working to get the pro-free trade message out to voters. Christopher Wenk, the U.S. Chamber of Commerce’s trade lobbyist, said his organization and other corporate allies are meeting with editorial boards and ramping up their own arguments just as the candidates ramp up the anti-trade rhetoric on the campaign trail.

“The bottom line is we did not properly sell NAFTA. We’re learning that now,” he said. “This is a broader issue for the business community. We do have a lot of work to do to stem this turn against trade across the country.”