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With Rick Santorum hot on his heels, Mitt Romney is launching a new, and somewhat puzzling, line of attack against his resurging GOP rival.

"Big Labor's Favorite Senator" read the bold headline on the Romney camp's latest opposition research e-mail to reporters, which was rife with examples of Santorum's breaking Republican ranks to cozy up with unions.

But the unions Romney accuses Santorum of supporting don't feel the same way.

"There is no support for Rick Santorum in the labor movement," said Rick Bloomingdale, president of the Pennsylvania AFL-CIO. "That just shows how far right that this race has moved. The fact that Rick Santorum is being considered a moderate is the most absurd thing I've ever heard."

According to the AFL-CIO's yearly report card on Senate voting records, Santorum voted "right," or in support of labor unions, about 13 percent of the time in 2006 and 11 percent of the time in 2005, placing him on par with the majority of Senate Republicans.

"How can you even begin to believe that's supportive of labor?" Bloomingdale said of Santorum's voting record. "Did he give us a couple of votes throughout his career? Sure. But he voted against working families much more than he voted for them."

Jack Shea, president of the Allegheny County Labor Council, which represents union workers in and around Pittsburgh, said his group fought against Santorum during his House and Senate campaigns.

"We never considered Rick to be a friend of the worker, not at all," Shea said. "The only time Rick Santorum was close to blue collar was when he was putting his shirt on."

In the 16 years Santorum served in Congress, he supported pro-union measures a handful of times.

As a freshman senator in 1996, Santorum voted against a national "right to work" bill that would have enabled workers to opt-out of paying union dues.

He also voted in favor of barring companies from firing striking workers and supported a bill to re-affirm the Davis-Bacon Act, which ensures that construction workers on federal projects are paid at least as much as their private sector counterparts.

Santorum said he supported these pro-union measures out of respect to the standing laws of the state he was representing.

"I would change those laws within Pennsylvania, but I'm not going to have the federal government change the law for the state of Pennsylvania," Santorum said on Fox News Sunday in January. "You don't have the federal government impose those [laws] on the state when the state decided differently."

As Romney works to paint Santorum as a union supporter, he is portraying himself as the candidate that will take on "union bosses."

"I happen to believe that you can protect the interests of American taxpayers, and you can protect a great industry like automobiles without having to give in to the [United Auto Workers union], and I sure won't," Romney said Wednesday at a campaign stop in Michigan.

But Romney's plea to non-union voters is a precarious one. The similarly pro-"right to work" argument he made during his 2008 White House bid paid off, especially in Michigan where Romney and Santorum are both going all-in this time around.

While the 28 percent of unionized Michigan voters split evenly between Romney and his GOP rival John McCain in 2008, non-union households favored Romney by 11 points, helping him win the state.

But the opposite story unfolded in 2000 when McCain and then-rival George W. Bush split the non-union vote evenly and McCain was boosted to victory by the support of union voters, 61 percent of whom chose McCain compared to the 34 percent who chose Bush.