Propelled by his debate performances and the demise of Herman Cain’s candidacy, former House speaker Newt Gingrich sits atop the Republican presidential field in Iowa with a clear lead over his closest competitors, according to a new Washington Post-ABC News poll.

Gingrich, according to the survey, has advantages that extend well beyond the horse race that put him in an enviable position in the final weeks before the state’s Jan. 3 caucuses, which serve as the formal start of the long nominating season. On electability, empathy and handling the economy, he does as well as or better than former Massachusetts governor Mitt Romney, who has long been described as the nominal front-runner for the nomination, or Rep. Ron Paul (Tex.).

Gingrich’s emergence is the latest dramatic shift in the GOP contest that have made front-runners out of a series of contenders only to have them fall quickly back into the pack.

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Gingrich’s campaign was considered all but dead only a few months ago. The question now is whether he can capi­tal­ize on his current strengths to make himself a lasting contender.

His support in national polls has been steadily increasing, and a victory in Iowa would probably give him a boost in the next two states to hold elections: New Hampshire, where he has been closing in on Romney, and South Carolina. A separate poll of South Carolina voters, by Winthrop University, shows Gingrich ahead of Romney in that bellwether state 38 percent to 22 percent.

But Romney’s advisers think they have the money and infrastructure to wage a protracted campaign even if he loses two of the first three states. No Republican has done that in the modern era, but the rules are different this year because some states will distribute delegates proportionally.

With 33 percent support among likely caucus-goers in the new poll, Gingrich runs well ahead of his two main rivals, Romney and Paul, a libertarian whose passionate following and anti-government rhetoric have made him a durable force in the race. Both are at 18 percent.

But Iowa Republicans are far from decided. More than six in 10 potential caucus-goers say they could change their minds, and even among the likeliest attendees, fewer than half say they have definitely chosen a candidate.

Of the top three, Paul’s supporters are the most solid, followed by Gingrich’s and Romney’s.

Beyond the leaders, only Texas Gov. Rick Perry scores in the double digits, with 11 percent. Two candidates who have devoted as much or more time as any to the state — Rep. Michele Bachmann (Minn.) and former senator Rick Santorum (Pa.) — are in the single digits, at 8 percent and 7 percent, respectively. Running last among the major candidates is former Utah governor Jon Huntsman Jr., with 2 percent. Huntsman has staked his candidacy on a strong finish in New Hampshire.

The survey was conducted Wednesday through Sunday, with all interviews following the latest allegations of sexual impropriety against businessman Cain and the candidate’s declaration that he was “reassessing” his candidacy.

Cain, a onetime leader in Iowa polls, announced Saturday that he was suspending his campaign. About 4 percent of likely caucus-goers surveyed put Cain as their top choice, far below his tally in state polls just weeks before. (In the reported data here, these respondents were allocated to their second-most-preferred candidate.)

Whether Gingrich has more staying power than the others who rose and then fell is the biggest question hanging over the Republican race today. The former speaker has little organizational strength in Iowa and began airing television commercials Monday. Just 13 percent of potential caucus attendees say they have heard from Gingrich’s campaign, half the proportion who say they have heard from Bachmann’s.

At the same time, Gingrich’s surge to the lead stems from far more than Cain’s situation. Gingrich — who has received considerable attention for his performances in candidate debates, picks up 38 percent of the vote among likely attendees who have watched a debate and 22 percent among those who haven’t.

As elsewhere in the country, the economy and jobs are the dominant issues in Iowa, with 38 percent singling them out as their top voting concerns. Romney’s private-sector background works for him in Iowa: Sixty-one percent of likely caucus-goers say his business experience is a reason to support him.

But Romney trails even among those who prioritize the economy and finishes far below Gingrich — whose political experience gets extremely positive reviews — when it comes to having the best pedigree for the White House.

Among all those highlighting economic or budgetary concerns, Gingrich leads Romney by 14 percentage points. Overall, 31 percent of likely caucus-goers say they trust Gingrich the most to deal with economic issues, 21 percent say Ron Paul, and 20 percent say Romney.

Asked who has the best experience to be president, more than twice as many likely caucus-goers say Gingrich than Romney or any other candidate.

Gingrich also has an edge over Romney when it comes to empathy, values and standing up for what he believes in. On the basic test of electability, 29 percent of likely caucus-goers say Gingrich represents the Republicans’ best chance to defeat President Obama in 2012, while 24 percent say so of Romney.

In short, of six attributes tested in the poll, the only one where Gingrich is not first or in contention for first is on being the most honest and trustworthy. On this score, it’s Paul with 23 percent and Bachmann with 17 percent, followed by Gingrich with 13 and Romney with 12, among likely caucus-goers.

On immigration — where Gingrich and Romney squared off in a recent debate — 27 percent trust Gingrich the most to Romney’s 8 percent, with Perry and Paul in between, at 18 and 13 percent, respectively.

Paul’s overall standing in the poll, which is similar to other recent findings, underscores the degree to which he has evolved from a quirky outsider four years ago to a politician with a growing following in a party even more heavily focused on limiting the size of the government.

Paul is building an organization in Iowa and has advertised here as well. More than half of his supporters say they’ll definitely back him on Jan. 3, and among all likely caucus-goers, two-thirds say his views on government are a reason to support him.

Romney has been an infrequent visitor to Iowa this year, to the consternation of some Republican activists. He started airing television commercials just in the past week, and his advisers have indicated that he will increase his activity in the state as the caucuses near. He also plans to call in help from surrogates, starting Wednesday with an appearance by New Jersey Gov. Chris Christie.

One big hurdle for Romney may be health care: Nearly half of likely caucus-goers say his work on the issue while governor is a reason to oppose his candidacy. Among “strong” tea party supporters who are likely to attend the caucuses, 60 percent call Romney’s health-care legacy a reason not to back him.

Overall, Romney gets just 10 percent of the vote among solid tea party supporters, to Gingrich’s 45 percent, Paul’s 14 and Bachmann’s 13 percent.

There had been speculation among outside political strategists that Romney was quietly hoping to score a surprise victory in the caucuses, a solid set-up for the nomination given his continued — although eroding — strength in New Hampshire. But the new poll underscores the risks to Romney of finishing poorly in Iowa, with potential collateral damage to his candidacy.

The poll was conducted Nov. 30 to Dec. 4 among a random sample of 858 potential Iowa Republican caucus-goers, including 356 most likely to participate. The margin of sampling error for potential voters is plus or minus four percentage points; it is six points for likely voters.

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