Three freshman Republican congressmen are joining the call by conservative leaders for Mitt Romney's opponents to lay off his record at private equity firm Bain Capital.

Republican Reps. Cory GardnerCory Scott GardnerDemocrats brace for nail-biting finish to Senate battle Trump expressed doubt to donors GOP can hold Senate: report The Hill's Campaign Report: 2020 spending wars | Biden looks to clean up oil comments | Debate ratings are in MORE (Colo.), Ben Quayle (Ariz.) and Adam Kinzinger (Ill.) sent a letter on Friday to all six major GOP presidential candidates, including Romney, imploring them to change tactics.

"We are alarmed at the recent attacks on free market capitalism some campaigns have decided to pursue. For the good of the country and conservative values, these attacks must come to an end," the three freshmen wrote.

While not mentioning Romney or Bain Capital by name, the letter was clearly aimed at attacks, primarily from Newt Gingrich and Rick Perry, that have faulted Romney for heading a company that flipped ailing companies and returned them to profitability, sometimes by shuttering facilities and laying off workers.

Democrats have been all too eager to see their GOP counterparts embrace the same line of attack they have already used against Romney and plan to carry through to the general election if he is the nominee. But conservative groups and commentators — including many who have been harsh critics of Romney in the past — have come to his defense, arguing that Republicans should never be in the position of undermining capitalism or bolstering Democratic talking points.

"We already have a president who is making it increasingly hostile for businesses to operate in this nation. Our presidential candidates should not be doing the same," Kinzinger, Quayle and Gardner wrote. "Class warfare is a tactic that President Obama has used to divide America and campaign against Republicans. Republican candidates for president should not be stooping to the same level."

Not all of them have. Ron Paul and Jon Huntsman have steered clear of hitting Romney on his business record. Paul even defended Romney over what he said was an out-of-context quote from a campaign event in New Hampshire on Monday, when Romney said he liked "being able to fire people who provide services to me."

The three House members argued that free markets have lifted far more people out of poverty than government programs, and that Republicans shouldn't isolate contrary examples to use for thir own political purposes.

"This is the America we believe in," they wrote. "Please stop attacking it."