DES MOINES, Iowa -- If Mitt Romney wins the Republican nomination for president, he'll face the urgent task of inspiring the party's conservative core and rallying them to beat President Barack Obama.

Judging by his performances in the primaries and caucuses so far, and the challenge he faces next week, he's got his work cut out for him.

Even Republicans who think he'll be the nominee worry about whether he can generate the intensity required to beat the Democratic incumbent.

These party leaders and activists, from the states voting Feb. 28 and the most contested ones ahead in the fall, say Romney has made strides toward addressing this problem. But, they say, he needs to do more to convince the Republican base that he's running to fundamentally reverse the nation's course, not simply manage what they see as the federal government's mess.

"I think Romney will be the nominee, but there is still tremendous work to be done," said Sally Bradshaw, a Florida Republican and adviser to former Gov. Jeb Bush. "He has got to find a way to unify the party and increase the intensity of support for him among voters who have supported Newt Gingrich, or Rick Santorum or Ron Paul or someone else. And that is going to be the key to how he does in the fall."

Romney leads in the delegate count for the nomination, and by a wide margin in private polling ahead of the Arizona primary Feb. 28.

But the rising challenge from former Pennsylvania Sen. Rick Santorum in the contest also that day in Michigan, where Romney was born and raised, underscores doubts about Romney's ability to ignite fervor in the GOP base.

He nearly tied Santorum in Iowa, although entrance polls showed that more of Santorum's backers than Romney's said they were strongly behind their chosen candidate.

Romney lost the primary in South Carolina last month to former House Speaker Newt Gingrich. More of Romney's supporters in that state said they would back him with reservations in the general election than would support him enthusiastically.

Santorum swept caucuses Feb. 7 in Colorado and Minnesota and the nonbinding Missouri primary.

Romney's challengers have risen by sounding more conservative and displaying sharper differences with Obama, while nipping Romney's appeal as the most electable against Obama.