WASHINGTON — Only 36 percent of registered voters say they’d definitely vote for President Barack Obama next year — but he still tops all Republican challengers in one-on-one matchups, according to a new McClatchy-Marist poll.

The survey also found that Republicans and Republican-leaning independents remain highly uncertain about who they want to face Obama. Former Massachusetts Gov. Mitt Romney leads the field, but with only 19 percent.

The findings “”speak to the vulnerability on the part of the president,”” but also doubts about the Republican field, said Lee Miringoff, the director of the Marist Institute for Public Opinion, which conducted the poll.

The survey was taken June 15-23, after seven Republicans held a nationally televised debate in New Hampshire on June 13. It also covered a period of low consumer confidence in the economy and gasoline prices near $4 a gallon.

The survey included 801 registered voters, with 308 Republicans or GOP-leaning independents. The error margin was plus or minus 3.5 percentage points for registered voters and plus or minus 5.5 percentage points for questions asked only of Republicans and Republican-leaning independents.

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, Miringoff said, “”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,”” Miringoff said, as doubts continue about Romney’s 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 on Monday, at 8 percent. Other candidates trailed: Minnesota Gov. Tim Pawlenty, Texas Rep. Ron Paul and businessman Herman Cain tied at 5 percent.

Two candidates scored 2 percent each: former House Speaker Newt Gingrich and former Utah Gov. Jon Huntsman, who announced his candidacy last week and is trying to position himself as a moderate-conservative alternative to Romney. Former Pennsylvania Sen. Rick Santorum got 1 percent, while former New Mexico Gov. Gary Johnson and political activist Fred Karger each had less than 1 percent. The poll also found 15 percent were undecided.

None would beat Obama today.

Romney comes closest, losing 46-42 percent, but he has lost some ground; in April, Obama led Romney, 46-45.

In other matchups, Obama beats Giuliani 48-41 percent; Bachmann 49-37 percent; Perry 48-39 percent; Pawlenty 47-33 percent; and Palin 56-30 percent.

The poll suggests that the Tea Party, the grassroots conservative movement that helped elect dozens of Republicans to Congress last year, has limited influence. Only 8 percent of registered voters said they strongly support the movement and 16 percent said they support it. Among Republicans, 45 percent said they supported the Tea Party, while 39 percent didn’t and 15 percent were unsure.

And 20 percent of Republicans and GOP-leaning independents said they were more likely to support a Tea Party-backed candidate, while 70 percent said it made no difference.

“”The Tea Party gives the Republican Party a lot of energy,”” Miringoff said. “”But for rank-and-file Republicans, it’s not the be-all and end-all.””