Craig Ruttle/AP Photo

Newt Gingrich has leapt to a sizable lead in preferences for the Iowa Republican caucuses, drawing on a rally from conservatives, positive views of his political experience and a sense he best represents "core Republican values" to push Mitt Romney into a trailing tie with Ron Paul.

Gingrich also scores evenly with Romney as the candidate best able to defeat Barack Obama, a mantle Romney long has sought. And this ABC News/Washington Post poll finds Gingrich ahead of the GOP field in trust to handle the economy, the top issue in Iowa, as it is nationally.

Given these views, 33 percent of likely Republican caucus-goers currently favor Gingrich for the GOP nomination, with 18 percent apiece for Romney and Paul. Rick Perry garners 11 percent support; Michele Bachmann, 8; Rick Santorum, 7; and Jon Huntsman, 2 percent.

There's room for movement. Just over half of likely caucus-goers, 52 percent, say there's a chance they may yet change their minds before the Jan. 3 event. Indeed, about one in four, 27 percent, say there's a "good chance" they'll switch their first preference - more than enough to shift the standings if the bulk of them were to move in the same direction.

In Iowa, as nationally, Gingrich's fortunes reflect the restlessness of conservative Republicans as they've sought an alternative to Romney. Support for Romney craters to just 11 percent among "very" conservative voters - a dominant group in the caucuses, accounting for more than four in 10 likely participants. Romney inches to 19 percent support among "somewhat" conservatives in Iowa, still trailing Gingrich. Only among moderates (and the few liberals) does Romney pull into competition. But they're far too few in number to make up for his conservative shortfall.

Romney's also vulnerable in another area: He faces a significant loyalty gap, in which his supporters are disproportionately likely to say they may change their minds. Paul's backers, by contrast, are highly loyal; Gingrich's fall between the two.

ADVANTAGE - This poll, produced for ABC News by Langer Research Associates, suggests that Gingrich holds the advantage to some extent because he lacks the negatives that are pulling back his top two rivals - for Romney, his record on health care and weak ratings on standing up for his beliefs; for Paul, his isolationist views and doubts about his electability.

All three candidates have attributes that make them popular - Gingrich his political experience (70 percent call it a major reason to support him), Paul his views on limited government (66 percent call this a strong draw) and Romney his business experience (61 percent say it's a major positive). But 45 percent also see Romney's legacy of mandatory health care in Massachusetts as a major reason to oppose him for the nomination, and 46 percent say the same about Paul's opposition to U.S. military interventions overseas.

Gingrich, by contrast, has no negative approaching that magnitude among likely caucus-goers in this survey - neither his views on immigration nor his marital history, seen as major shortcomings by just 15 and 16 percent, respectively.

Slightly fewer than four in 10 likely caucus-goers call the economy and jobs the most important issue in their vote; for nearly three in 10 more, it's the federal budget deficit. Gingrich leads in both groups, but Romney's second among economy voters, while Paul's No. 2 among those focused on the deficit.

ATTRIBUTES and ISSUES - On personal attributes, Gingrich and Paul alike lead Romney by 2-1 margins as the candidate who's most likely to "stand up for what he or she believes" -  suggesting resonance in the charges that Romney's been inconsistent on the issues. Both similarly lead Romney on empathy, as the candidate who "best understands the problems of people like you." Indeed, on these attributes Romney is numerically in fourth place, also behind Bachmann (albeit not by a statistically significant margin).

Gingrich, meanwhile, walks away with views that he's got the best experience to be president: 43 percent say so, followed by a vastly lower 16 percent for Romney, 13 percent for Paul. And 29 percent pick Gingrich as having the best chance to defeat Obama. Twenty-four percent give Romney the best shot; Paul lags badly here, with just 8 percent.

Gingrich, as noted, leads in trust to handle the economy, selected by 31 percent of likely caucus-goers, with Paul and Romney running essentially evenly behind him, at 21 and 20 percent, respectively. Gingrich also leads in trust to handle immigration, perhaps surprisingly, given his less orthodox views on the issue. He's weaker in trust to handle social issues such as abortion and gay marriage; on this more Iowans prefer Bachmann over the rest of the GOP field.

Paul, for his part, leads Gingrich and Romney as the "most honest and trustworthy" candidate, and, as noted, runs evenly with Romney in trust to handle the economy, behind Gingrich. Paul's support, however, is disproportionately high among younger Iowans, a group whose turnout always is a question mark, especially at the historically lightly attended caucuses. Romney, by contrast, does better with seniors - again as has been the case in national polls - a more reliable turnout group.

While turnout is a wild card, Gingrich leads not only among likely caucus-goers, but also among a larger group, potential caucus participants. In this population Gingrich has 28 percent support; Romney, 18 percent; Paul, 16; Perry, 12; Bachmann, 10, Santorum, 4; and Huntsman, 2 percent.

Gingrich's rise follows the implosion of Herman Cain's campaign; he withdrew from the race Saturday following claims Nov. 28 by a single mother, Ginger White, that they'd had a 13-year affair. (Cain denied it.) This survey, the first conducted entirely after White's charges were made public, computed vote preference by allocating Cain's supporters to their second choice.

When Cain is included as a candidate, he has just 4 percent support from likely caucus-goers, a complete collapse from his 23 percent in a Des Moines Register poll in late October. National polls have tracked the same drop-off for Cain, as well as, earlier, short-lived GOP rallies for Bachmann and Perry. The question for Gingrich is whether he can hold that ground.

In one measure of candidate potential, relatively few likely caucus-goers rule out any of the candidates. Just under a quarter, 23 percent, say they definitely would not support Bachmann for the nomination; 18 percent say the same about Romney, 16 percent about Paul. Significant majorities, then, are not flatly rejecting any of these or the other candidates. Notably, among very conservatives, just 23 percent rule out Romney, meaning he may still have a chance in this group were Gingrich to stumble badly in the weeks ahead.

Bachmann's problems seem to focus on ability, not likeability. She's rated competitively on qualities such as empathy, honesty, GOP values and standing up for her beliefs, and, as noted, on dealing with social issues. But she falls far short on handling the economy and immigration, having the best experience and in being seen as best able to defeat Obama in November.

RELIGION - Religious belief was a strong factor in 2008, when evangelical Christians in Iowa flocked to Mike Huckabee, and enough expressed concerns about Romney's Mormon beliefs to make a difference. It looks like less of an issue now. Some likely caucus-goers, 15 percent, call Romney's religion a major reason to oppose him; 4 percent call it a major reason to support him, while 78 percent say it's not a major factor either way. Gingrich, in any case, leads across religious groups. Ideology, issues and attributes look like much bigger differentiators.

OUTREACH and ATTENTION - One surprising result is that Gingrich leads in Iowa despite a shortfall in personal outreach. Three in 10 likely caucus-goers say they have been contacted in person by a representative of the Romney, Paul, Perry and/or Bachmann campaigns. Fewer, 19 percent, say they've been contacted by the Gingrich campaign, similar to Santorum (15 percent) and ahead only of Huntsman (7 percent.) Yet Gingrich leads - raising the question of whether Iowa's reputation as a bastion of retail politics is entirely deserved.

Attention to the contest is substantial: Among all potential Republican caucus-goers, 83 percent say they're following the campaign closely; among likely caucus-goers this jumps to 94 percent. Seventy-six percent of likely caucus-goers also describe themselves as satisfied with their choice of candidates (compared with 63 percent of all potential caucus-goers.) And 92 percent of likely caucus goers describe themselves as enthusiastic about the candidate they currently support. Many fewer, though, are "very" enthusiastic, 36 percent, 9 points fewer than at this time four years ago.

The debates have attracted substantial audiences, with 68 percent of likely-caucus goers (and 53 percent of potential participants) saying they've watched. Gingrich's advantage expands among debate watchers, as well as among those who are very closely following the campaign.

A WORD ON TURNOUT - Turnout in the Republican Iowa caucuses is usually about a mere 5 percent of the adult population - about 120,000 people in 2008, or roughly the capacity of Michigan Stadium. That makes identifying likely caucus-goers difficult.

This survey was conducted via random-sample telephone interviews (landline and cell phone alike) of the full Iowa adult population, winnowing down both to potential caucus-goers, then further identifying likely participants. Potential caucus-goers account for 21 percent of the general population; likely caucus-goers, 8 percent.

METHODOLOGY - This ABC News/Washington Post survey was conducted by landline and cell phone Nov. 30-Dec. 4, 2011, among a random sample of 858 potential and 356 likely Iowa Republican caucus-goers. The results have a margin of sampling error of plus or minus 4 points for potential caucus-goers and 6 points for likely participants. This survey was produced for ABC News by Langer Research Associates of New York, N.Y., with sampling, data collection and tabulation by Abt-SRBI of New York, N.Y.