WASHINGTON (AP) — Mitt Romney's surprisingly easy rise to the top of Iowa presidential polls, aided by his GOP rivals' in-fighting, masks vulnerabilities he will have to confront eventually.

Even if he wins Tuesday's caucus, Romney has yet to excite his party's restless conservative base. And his rivals' focus on each other has let Romney skate along with minimal focus on his contentious health care record and his changed positions on abortion, gay rights and other issues since his days as Massachusetts governor.

Groups friendly to President Barack Obama plan to use the "flip-flopper" label against Romney if he's the nominee. They hope it will damage him as much as it did Democrat John Kerry in 2004.

Romney has run a smart, nearly mistake-free campaign so far. But his string of luck and efficiency might have a dark lining. He hasn't been forced to hone incisive answers to tough questions certain to rise in debates if he faces only one or two remaining GOP opponents, or Obama.

Romney appears testy on the rare times when pressed about his policy shifts and his Massachusetts program that required residents to obtain health insurance. Democrats enjoy replaying a Nov. 30 Fox News interview of Romney expressing irritation at such questions.

But Romney has benefitted from a relatively weak field of rivals who, from the start, mostly criticized each other in hopes of becoming the party's conservative alternative.

Those candidates' various problems have left them "too weak to exploit Mitt Romney's vulnerabilities," which include his tenure at a corporate takeover firm and "the number of times he has reinvented his positions," said Jen Psaki, a former Obama White House aide. "But regardless of when the field narrows," she said, "there will be plenty of time to explore his record in more depth."

Former House Speaker Newt Gingrich has all but conceded that Romney will out-perform him, and perhaps everyone else, in the Iowa caucuses. But Gingrich, the victim of withering TV attacks funded by Romney allies, says Romney will come under hotter fire in New Hampshire, whose GOP primary is Jan. 10.

"New Hampshire is the perfect state to have a debate over Romneycare and to have a debate about tax-paid abortions, which he signed," Gingrich told reporters Sunday in Iowa. "And to have a debate about appointing liberal judges, which he did."

It's not clear that Gingrich will have enough money to wage an effective campaign against Romney in New Hampshire or South Carolina, which votes on Jan. 21. But Romney's consistent failure to galvanize the GOP base leaves a door open for the others to keep asking conservatives to coalesce around an alternative.

If that fails, and Romney becomes the nominee, then Democrats can hope right-wing disenchantment will depress Republican turnout in November. Many Republicans scoff at the idea. Animosity toward Obama and his economic record will unite conservatives, they say.

Playing off that idea Monday, former Sen. Rick Santorum urged Iowa's undecided Republicans to nominate a true conservative, himself. Alluding to Romney, Santorum implored voters to "not settle for someone as your nominee who might be able to win the election but it might be a pyrrhic victory."

In the type of second-tier squabbling that has shaped the campaign for months, Romney didn't need to counter-attack Santorum by himself. Texas Gov. Rick Perry called Santorum a weak candidate who can't defeat Obama. On MSNBC, Perry noted that Santorum badly lost his 2006 Senate re-election bid in Pennsylvania, proving "he can't win races when it matters against a liberal Democrat."

The Des Moines Register's final poll of Iowa Republicans highlights Romney's inability thus far to attract a large and enthusiastic plurality of GOP voters, let alone a majority. One in four likely caucus-goers named Romney as their top pick, essentially tying him with Rep. Ron Paul of Texas.

Romney held an overwhelming lead on one key question: who is most electable next fall. He trailed far behind Gingrich on "who is most knowledgeable." And Iowa Republicans ranked Romney fifth when asked who is "best at relating to ordinary Iowans."

With Romney's Republican rivals doing little to hit his soft spots, Obama's supporters are stepping up. Democratic officials moved into Iowa over the weekend for a host of events and campaign videos aimed at Romney.

Speakers included Randy Johnson, who worked at an Indiana paper plant bought in 1992 by Bain Capital, a corporate reorganization firm that Romney headed. Johnson, one of the workers laid off when the plant closed after Bain took on heavy debts, said Romney seemed more concerned about profits than employees.

Romney faced similar charges in 1994, when he ran unsuccessfully for the Senate from Massachusetts. His campaign says Bain Capital had many success stories and a handful of failures in its efforts to make companies more efficient and competitive.

Obama campaign aides have expressed disappointment that the GOP presidential candidates have done so little to capitalize on the Bain record and Romney's policy switches.

The flip-flop issue might be less useful to Obama than to Romney's Republican rivals, who are trying to woo staunchly conservative voters in the primary. By reminding general election voters that Romney once supported legalized abortion and gun control, Obama might make the former Massachusetts governor more attractive to all-important independent voters.

If that happens, Romney's campaign savvy and luck will prove long-lasting.