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What’s with Mitt Romney?

He’s been the constant in an otherwise shifting Republican landscape, the steady leader or co-leader of the field, the standout choice in ratings of electability — yet with weaknesses in core GOP groups, shortfalls in views of his personal attributes and no apparent momentum.

And then there’s that little matter of the Massachusetts health care law.

It all adds up to a conundrum of a candidacy. In a year when the Democratic incumbent clearly is vulnerable, 33 percent of Republicans and Republican-leaning independents pick Romney as the GOP candidate most likely to defeat Barack Obama in the 2012 election, a dozen points above his closest competitor. Yet fewer, 24 percent, support him for the nomination, basically steady the past three months, and slightly down from his peak support, 30 percent, in July.

The question for Romney, who’s scheduled to visit Iowa today, is whether electability is enough. Compared with his rating on beating Obama, just half as many leaned Republicans in this ABC News/Washington Post poll, 17 percent, see Romney as the most honest and trustworthy; 22 percent instead pick Herman Cain. On best reflecting core Republican values, 20 percent choose Romney, but essentially as many, 19 percent, go for Newt Gingrich.

Nor does Romney clearly lead on other attributes — being “closest to you on the issues” (on which he runs evenly with Cain), best understanding the problems of people like you (again just 17 percent for Romney, vs. 21 percent for Cain) or even on Romney’s supposed strong suit, being best able to handle the economy (22 percent, vs. 19 percent for Cain).

Romney’s religion does not appear to be a major impediment; 77 percent of leaned Republicans say his being a Mormon makes no difference in their vote choice, while 20 percent say it makes them less likely to support him. Compunctions about voting for a Mormon started much higher, 36 percent, early in the 2008 contest, but subsided over that year to the same level as today.

Romney’s role in enacting mandatory health care while governor of Massachusetts, however, is another matter: Far more potential Republican voters, 48 percent, say this makes them less likely to back him, including a third much less likely. Seniors and lower-income Republicans stand out as more critical of Romney on this issue, as do conservatives compared with moderates.

Ideological acceptability, indeed, may be Romney’s greatest challenge. This poll, produced for ABC by Langer Research Associates, finds that just 13 percent of conservatives say Romney best understands the problems of people like them, 16 percent say he’s closest to them on the issues and 16 percent say he best reflects the GOP’s core values. Among moderates these rise to 25, 27 and 29 percent, respectively.

Romney’s overall support for the nomination, in turn, declines from 31 percent among moderates to 21 percent among conservatives and 15 percent among leaned Republicans who describe themselves as very conservative — a group accounting for nearly three in 10 potential GOP voters. Romney wins just half of Cain’s 30 percent support among very conservatives and only manages to run alongside both Gingrich and Perry, at 17 percent each, in this group.

For all that, the notion that Romney simply can’t break through doesn’t bear out: In fact he leads as the second choice among Republicans and Republican-leaners who currently favor other candidates. In this group, 27 percent identify Romney as their backup.

The actual equation, of course, depends on who stays in the race, who goes, and how other dynamics play out, in a contest in which, among those with a current preference, 69 percent say they yet may change their minds. While Romney, for instance, has a numerical lead for second choice among Cain supporters, Cain has a numerical lead for second choice among Romney supporters. Sliced another way, among those who currently support neither Romney, Cain nor Perry, 30 percent pick Romney as their second choice, 22 percent Cain, 15 percent Perry.

While “last man standing” may not be as straightforward a path as he might prefer, Romney’s acceptability as a second choice suggests a possible route for him to the nomination — assuming that other candidates do drop out over time, and that he, in fact, can win over their supporters.

METHODOLOGY – This ABC News/Washington Post poll was conducted by telephone Oct. 31-Nov. 3, 2011, among a random national sample of 1,004 adults, including landline and cell-phone-only respondents, and 438 leaned Republicans. Results have a margin of sampling error of 3.5 points for the full sample and 5.5 points for leaned Republicans. The 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.