Former Massachusetts governor Mitt Romney looks on as former House Speaker Newt Gingrich, speaks during a Republican presidential debate in Sioux City, Iowa, Dec. 15, 2011. (Eric Gay/AP)

Two weeks before Iowans cast the first votes of the 2012 election cycle, Republicans nationally are sharply divided over whom their party’s presidential nominee should be, with Mitt Romney and Newt Gingrich locked in a dead-even race, according to a new Washington Post-ABC News poll.

Gingrich and Romney are each favored by 30 percent of Republicans and Republican-leaning independents. Running behind them is Rep. Ron Paul (Tex.), whose libertarian philosophy has attracted a strong following. He stands at 15 percent, about double his tally in an early November poll. All other active candidates are in the single digits.

The survey shows President Obama receiving his highest approval rating since March, with the exception of a brief rise after the killing of Osama bin Laden. Although most Americans continue to disapprove of the president’s performance on the economy, the number who disapprove of his overall performance has dipped below 50 percent for the first time this fall.

Romney has edged higher in the GOP contest, breaking out of the 20s for the first time since July. Gingrich has jumped greatly since early November, in part because of businessman Herman Cain’s decision to suspend his candidacy.

Gingrich peaked at an even higher level and started to fade in other national surveys conducted between the Post-ABC polls, suggesting that he may have lost some momentum recently. He was on the defensive in the final debates before the Jan. 3 Iowa caucuses and has come under sharp attack by Romney in a series of media interviews. He also is the target of hundreds of thousands of dollars in negative ads in Iowa from his major rivals and their allies, all of which could be taking a toll.

Romney has a 10-point advantage on the key question of who Republicans think is most likely to beat Obama. Romney also leads Gingrich when it comes to dealing with issue No. 1, the economy. Gingrich counters with a big advantage on experience: By about 2 to 1, more see him as having the best résumé for the White House and as being the most qualified to be commander in chief.

In other ways, the two are evenly matched. They are tied at 23 percent on which best reflects the core values of the Republican Party. They run about equal on which “is closer to you on issues.” Romney and Gingrich are also tied among those who are the strongest supporters of the tea party movement, and draw almost evenly among men and women and across age groups.

But there are differences. Gingrich has a wide advantage — 36 percent to 22 percent — over Romney among those who consider themselves “very conservative.” The former House speaker edges Romney, a former Massachusetts governor, among those without college degrees and among those with annual household incomes under $75,000. Romney has small advantages among those with more formal education and higher incomes.

Although a big majority of Republicans and GOP-leaning independents across the country say they support either Romney or Gingrich, only about a third of potential voters say they are firmly committed to any one contender. Many of those voters pick someone other than Romney or Gingrich when asked about the candidates’ attributes and their capacity to handle key issues.

This year — perhaps because of the heavy influence of more than a dozen televised candidate debates — national polls have tracked closely with state surveys. That could change soon as campaigning intensifies in Iowa and New Hampshire, where voters are encountering a much heavier volume of television attack ads, direct mail and face-to-face campaigning than voters elsewhere.

But the new national survey provides a variety of insight on how Republicans are evaluating their leading candidates.

Both Gingrich and Romney can point to their backgrounds as reasons Republicans should back them. More than six in 10 consider Gingrich’s political experience a major reason to support him, and more than seven in 10 say they approve of the work he did as House speaker in the 1990s.

But Gingrich’s experience as a private citizen since leaving Congress draws negative responses. His work as a consultant for Freddie Mac has come under blistering attack from his GOP rivals in recent debates and on Iowa airwaves.

By 44 percent to 33 percent, Republicans surveyed say they have an unfavorable view of the political work he has done since leaving office. About three in 10 of his supporters have negative views of his activity since leaving the speaker’s office, and among GOP-leaning independents, 53 percent say they view that work unfavorably.

Gingrich has responded by saying that he has never lobbied and that most of the money from Freddie Mac went toward overhead and other expenses, not to him directly. On Monday, his campaign began an advertising counteroffensive.

Although more Republicans think Gingrich has the best experience to be president, Romney’s work in the private sector and as governor of Massachusetts are viewed positively. More than six in 10 say his business experience is a big reason to support him; a majority have a favorable impression of the work he did buying, restructuring and selling companies while at Bain Capital; and more than two-thirds say they approve of the job he did as governor.

Not all about that record in Massachusetts is viewed positively, however. About 28 percent consider his policies on health care — he signed into law a coverage mandate for all Massachusetts residents — a major reason to support his candidacy. But 36 percent say it is a major reason to oppose him.

Gingrich’s critics have questioned whether he has the temperament to be president. Romney called him “extremely unreliable” in a Washington Post interview a week ago and “zany” in a New York Times interview the next day.

In the Post-ABC News poll, 61 percent say the former speaker has the right personality and temperament to be president — an 11-point increase since June and a shade under the 67 percent who say so about Romney.

The other candidates are struggling for national attention. Texas Gov. Rick Perry and Rep. Michele Bachmann (Minn.) are at 7 percent in the poll, while former senator Rick Santorum (Pa.) and former Utah governor Jon Huntsman Jr. are at 3 percent.

Most Republicans say Paul’s views on limited government are a major reason to support him, while many consider his opposition to U.S. military intervention a big reason to oppose his candidacy. Nearly two-thirds say they can rely on Paul to say what he really believes — more than say so about Gingrich or Romney — but a slim majority says he lacks the personality and temperament to be an effective president. Nearly half say that his policies as president probably would be unacceptable to most Americans.

Polling manager Peyton M. Craighill and polling analyst Scott Clement contributed to this report.