OSKALOOSA, Iowa — Presidential candidates are again marshaling their forces in Iowa, which is up for grabs in November. Mitt Romney, eager to capture a state President Obama carried four years ago, is planning another visit on Tuesday as he bulks up his 11 field offices to compete with Mr. Obama’s 17.


But as he strides toward the traditional show of party unity at the Republican convention this month, Mr. Romney faces a worrisome undercurrent here: that the grass-roots elements who animated the Iowa caucuses — including evangelical Christians, Ron Paul supporters and Tea Party members — are not fully behind his candidacy in a battle that will be determined partly on who turns out his party’s base.


“He just doesn’t seem to connect well, and I’m not sure he’s a strong enough candidate, to be very, very honest,” said Steve Boender, a farmer here in southeast Iowa who supported Rick Santorum in the state caucus. “I’m probably going to hold my nose and vote for him,” Mr. Boender added, “but I’m afraid there are a fair amount of people that will” sit on their hands.


Prominent conservatives have called on Mr. Romney to make bolder efforts to rally the Republican base, including Gov. Scott Walker of Wisconsin and Sarah Palin, who pleaded with him last month to “light our hair on fire.”


And Tuesday’s victory of Ted Cruz, an insurgent with Tea Party support who vanquished Texas’s lieutenant governor in a Senate primary, raised new questions about whether a Republican Party deeply split over its future direction and leadership can fully unite in November behind an establishment politician.


There is little doubt that conservatives want to drive Mr. Obama from office. But whether Mr. Romney, who once backed universal health care and supported abortion rights, can generate the excitement needed to draw these voters to the polls is a question that worries some Republicans.


“Cruz won because he was viewed as the change agent,” said Craig Robinson, a former political director of the Iowa Republican Party. He faulted the Romney campaign for playing small ball for much of July by focusing on a statement the president made about who deserves credit for a business’s success.


“While Boston is busy attacking President Obama for the remarks he made in Roanoke a few weeks ago, the electorate is clamoring for a candidate who will attack and reform the federal government,” Mr. Robinson said, alluding to the headquarters of the Romney campaign.


Many Republican strategists, including in the Romney campaign, insist that opposition to Mr. Obama alone will rally the base. But with fewer than 100 days before the election, some voter surveys highlight an enthusiasm gap Mr. Romney faces. A Quinnipiac University/New York Times/CBS News poll this week of likely voters in three swing states — Florida, Ohio and Pennsylvania — found supporters who “strongly favored” Mr. Romney trailing Obama supporters who “strongly favored” the president by double digits.


The Romney campaign disputes the significance of such findings, arguing that dislike for Mr. Obama is most intense among conservatives, who are most likely to vote.  


“Intensity drives turnout,” said Neil Newhouse, Mr. Romney’s pollster. “Every measure shows Republicans and conservatives are more intense in their opposition to President Obama than Democrats are in support of him. They will be there in November.”


Mr. Romney and his top strategists are running a textbook general-election campaign, in which the candidate tacks away from the base to appeal to undecided voters in the middle.


Some conservative leaders echoed the warnings of Mr. Santorum and Newt Gingrich from the nominating fight that when Republicans choose an establishment figure, recent history has not been kind.


“There’s no doubt the Tea Party groups, the social conservatives, evangelicals, fiscal conservatives — they’re all interested in getting rid of Obama,” said Bob Vander Plaats, a leader of social conservatives in Iowa. “And yet there is a reluctance to wholly embrace Romney at this point. That’s a concern, and it’s a concern of a high enough magnitude that all you have to do is ask John McCain and Bob Dole, how did that turn out for you?”


Although Mr. Romney came within a whisker of winning the Iowa caucuses in January, he was not nearly as competitive in rural regions of the state like the southeast.


Mr. Romney “should be leading by a lot at this point in the game,” said Mark Doland, a member of the Iowa Republican Party’s central committee from Oskaloosa. Mr. Romney, he said, has failed to excite conservatives by speaking out on issues like same-sex marriage and slashing the federal government.