America's doctors overwhelmingly back Mitt Romney for president, with sole practitioners giving the former Massachusetts governor a huge two-to-one edge over President Obama.

In a new poll of 4,681 physicians provided to Secrets, the doctors favored Romney 53 percent to 33 percent for Obama, with 12 percent undecided and the rest backing a third candidate.

But sole practitioners polled from the MDLinx doctor base were quizzed about the candidates, 58 percent chose Romney to 28 percent for Obama, an indication that private doctors are worried about the impact Obamacare will have on their practices.

By comparison, doctors working for hospitals backed the president 47 percent to 37 percent.

"Healthcare has charged to the top of the campaigns' messaging this year, and U.S. physicians have a lot invested in the direction taken in the next administration," said Stephen Smith, of MDLinx. "While there were several observed trends, the most substantial was that as the size of the physician's workplace fell from hospital-based to solo practitioner, each step down in size showed an additional shift toward the Republican candidate."

In the survey, many doctors backed changes in health care. But those with small practices worried that Obamacare will put them out of business.

"I think if Obama goes through with his plan...there will be no doctors," one physician told MDLinx. "No one is going to take on that liability if they are only paid as much as their nurse."