Mitt Romney holds a strong lead nationally in the race for the Republican presidential nomination, with a 2 to 1 advantage over his closest competitors, according to a new Washington Post-ABC News poll.

Romney won the first two contests in the nominating process, and none of the other candidates has been able to demonstrate the broad, sustained support among the party’s conservative base that would translate into a successful challenge. An overwhelming majority of Republicans predict that he will be President Obama’s opponent in November.

The poll highlights the significance of Saturday’s primary in South Carolina, which could be the last chance for Romney’s main opponents to halt his steady march toward the nomination. A loss by the former Massachusetts governor, however, could change the dynamic of the contest by helping to catapult one of his rivals into a head-to-head matchup in Florida’s primary on Jan. 31.

Romney wins the support of 35 percent of all Republicans and GOP-leaners nationwide, with former House speaker Newt Gingrich (Ga.) and Rep. Ron Paul (Tex.) neck and neck with about half the support Romney enjoys. Former senator Rick Santorum (Pa.), who surged to a second-place finish in the Iowa caucuses, checks in at 13 percent, his highest level of the campaign. Texas Gov. Rick Perry runs fifth, at 9 percent, rounding out a field that shrank Monday with the decision by former Utah governor Jon Huntsman Jr. to quit the race and endorse Romney.

Romney has edged up since a Post-ABC poll in mid-December before the campaign’s first votes were cast in Iowa and New Hampshire, boosted by a sharp increase in his perceived electability and a steep falloff in national support for Gingrich.

With his rivals closely bunched and well behind him, Romney has obvious advantages, reinforcing an aura of inevitability that surrounds his candidacy. Nearly six in 10 Republicans see him as the party’s best shot at defeating Obama.

Just a month ago, Romney held a 10-point advantage over Gingrich as the most electable candidate in the field. That lead has swelled to a whopping margin of 57 percent to 10 percent. In the intervening weeks, millions of dollars in negative ads hit the former speaker in Iowa as his support plummeted there and nationally. Gingrich, who declared late last year that he was likely to be the nominee, finished fourth in Iowa and New Hampshire.

Overwhelmingly, Republicans expect Romney to be the party’s nominee. Even about two-thirds of those who do not support him for the nomination see him as the one most likely to be atop the party’s presidential ticket.

Romney also has consolidated support on issue No. 1: the economy. For the first time in the campaign, he has a double-digit lead over his rivals when it comes to dealing with economic issues; he beats everyone on this measure by 2 to 1. And he is up 13 points when it comes to handling the federal budget deficit. Exit polls in New Hampshire and Iowa showed overwhelming percentages of voters focused on these two issues.

But he has not done as well consolidating support on other important candidate attributes.

For example, about 21 percent consider Romney the most empathetic candidate, about the same as a month ago. About a quarter view him as the one who best reflects the core values of the GOP, but just as many say the same about Gingrich. here has also been little movement in the percentages of Republicans and GOP-leaning independents who single out Romney as the most honest and trustworthy or the most authentic.

Romney has faced a week of battering over his role at Bain Capital, a private-equity firm he helped found and where he made his fortune. Gingrich and Perry initially jumped on Romney for his role there, with Perry at one point describing Bain and other such firms as “vultures.” A super PAC supporting Gingrich has run ads attacking Romney and put out an online documentary casting Bain and Romney in a harsh light, although the accuracy of the claims has been called into question.

Most potential primary voters see Romney’s work buying, restructuring and selling companies in a generally favorable light, but since mid-December, there has been an increase in unfavorable views. Among Republicans, 34 percent have negative impressions of Romney’s private-sector work, up from 20 percent just a month ago.

Still, far more regard Romney’s private-sector work as doing more to create rather than cut jobs. And the opportunity may be closing for Romney’s rivals to capitalize on another potential vulnerability: his support for health-care reform as governor of Massachusetts. A plurality says his work in the area is not a major factor in their vote, with about a 20-point drop in the number of very conservative potential voters saying the Massachusetts experience is a big reason to oppose his candidacy.

And Romney’s rivals have their own vulnerabilities.

More than half, 51 percent, view Gingrich’s work as a political consultant in an unfavorable light, up seven points in the month. The negative turn here has been particularly steep among staunch conservatives, with a 12-point rise in unfavorable impressions.

About half of all potential voters see Paul’s strident opposition to U.S. military interventions overseas as a major reason to oppose him, and exactly half of possible voters say that, if elected, he would probably pursue policies that are unacceptable to most Americans.

In Iowa and New Hampshire, Santorum won over large numbers of voters by making abortion a priority issue, but nationally, his blanket opposition to the procedure draws negative reviews. Nearly two-thirds of all Republicans and GOP-leaning independents express unfavorable views of his stand on the issue. And nearly a third consider his support for earmarks while in the Senate a big reason to oppose his candidacy, double the proportion who see that as a major reason to back him.

The poll was conducted Jan. 12 to 15; the margin of sampling error for Republicans and GOP-leaning independents is plus or minus 5.5 percentage points.

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