Poll: Obama leads all potential GOP 2012 nominees

In a new poll by McClatchy-Marist, Obama faces determined opposition: 43 percent of voters said they would definitely vote against him in 2012. So would 43 percent of independents, 10 percent of Democrats and 85 percent of Republicans.

But none of his potential GOP rivals would beat him today. Republican candidates, said Miringoff, "have not at this point developed credibility with voters."

Romney maintained a similar level of support as he did in an April survey, despite the attention given his June 2 announcement and a strong debate performance on June 13.

He may have "hit a ceiling," said Miringoff, as doubts continue about his health care stand. As governor, Romney signed into a law a health care plan requiring near-universal coverage, a plan similar to the 2010 federal health care law that Republicans loathe. Romney has explained that each state should be able to decide what's best for its residents.

Three potential candidates who haven't declared their candidacies trailed Romney: Former New York City Mayor Rudolph Giuliani, 13 percent; Texas Gov. Rick Perry, 13 percent, and former Alaska Gov. Sarah Palin, 11 percent.

The poll surveyed Republicans and Republican-leaning independents. Giuliani and Palin enjoy widespread name recognition, but the lesser-known Perry's ascendance among them may suggest growing interest in him as he weighs whether to get in.

Next was Minnesota Rep. Michele Bachmann, who declared her candidacy Monday, at 8 percent. Other candidates trailed: Minnesota Gov. Tim Pawlenty, Texas Rep. Ron Paul and businessman Herman Cain tied at 5 percent.