Charles Krupa/AP Photo

Two days before New Hampshire voters head to the polls, the Mitt Romney campaign unleashed a trove of opposition research on former Pennsylvania senator Rick Santorum.

Before today, the campaign had only publicized comments made by new Romney surrogate Sen. John McCain, R-Ariz., regarding Santorum.

In a research document emailed to reporters during Sunday's NBC debate, the Romney camp knocks Santorum for "repeatedly" joining with Democrats and siding with labor unions.

The document blasts Santorum for supporting the Davis-Bacon Act, which mandates that construction workers under federal contracts must be paid at least as much as their privately contracted counterparts.

The conservative Heritage Foundation claims the act "unnecessarily inflate[s] the cost of federal construction projects."

Romney's campaign paper also criticizes Santorum's record in the Senate for preventing employers from hiring permanent replacements for striking workers and voting against a bill to end a railroad union strike in 1992.

This is the first time Romney has unleashed a written attack against Santorum and is perhaps a sign that the front-runner now sees Santorum as a serious contender as the campaign moves beyond New Hampshire, where Romney has a 36-point lead over Santorum.

But the paper spat between the two top finishers in the Iowa caucus was mild compared to the verbal spat that took place on the New Hampshire debate stage Sunday morning.

"If his record was so great as Governor of Massachusetts why didn't he run for reelection? If it was that great, why didn't you- why did you bail out?" Santorum said. "We want someone who's gonna stand up and fight for the conservative principles, not bail out and not run and not run to the left of Ted Kennedy."

Romney countered with this: "Run again? That would be about me. Now I have the opportunity, I believe, to use the experience I have- you- you got- a surprised look on your face."

After talking over each other back and forth, Romney continued: "Yeah, what I'm gonna tell you is I- this- this for me, politics, is not a career. For me my career was being in business and starting a business and making it success."