MUSCATINE, Iowa - Marco Rubio no doubt wants to sit behind the big desk in the Oval Office. What is not so clear is how hard he is willing to work to get there.

Republican activists - including many who appreciate Rubio's formidable political gifts and view him as the party's best hope for beating Democratic presidential front-runner Hillary Clinton - say they are alarmed at his seeming disdain for the day-to-day grind of retail politics. Even some staunch supporters are anxious.

"Rubio has not put in the face time that he really needs to have, I don't think," said Al Phillips, an influential South Carolina pastor who backs Rubio."I think that's been somewhat to his detriment."

That may be, as some of his allies fret privately, a sign of overconfidence in his own abilities. Or it may be a smart strategic decision that the personal touch is overrated in an era in which celebrity billionaire Donald Trump is leading the field with a campaign that consists largely of mega-rallies, barrages of tweets, and television interviews that are literally phoned in.

And Rubio is certainly capable of turning on the charm with one key constituency: deep-pocketed donors. He recently secured support from billionaire hedge-fund managers Paul Singer and Kenneth Griffin. He's believed to be the favorite to win over billionaire casino magnate Sheldon Adelson.

The first-term senator from Florida has many qualities that could help him win nationally: a compelling biography, youth, charisma, and intellect. He has shone in debates and is a sought-after guest on national television.

But connecting in the early states has traditionally been a different kind of endeavor, one in which voters expect to get to know the candidates on a one-on-one level.

Though Rubio has lately stepped up his schedule in Iowa, for instance, he has usually stayed close to the Des Moines metropolitan area. Republicans joke that he is running for "mayor of Ankeny," the suburb where his state headquarters are located.

At his town halls, Rubio generally answers no more than a handful of questions. Leaders of grass-roots organizations grumble that it is difficult to entice him to speak at their events. And Rubio has limited interaction with the news media who cover him.

"Is his campaign a little more superficial than that of other people in his bracket in New Hampshire? I think the answer is yes," said former New Hampshire Republican Party chairman Fergus Cullen, who is neutral in the race.

A top party official from a critical swing state who did not want to be identified on a topic of increasing sensitivity in GOP circles said that other contenders are becoming familiar faces to voters there. "The big question is, where is Marco Rubio? People have tried to get him to attend stuff, and he just hasn't," the state party leader said.

Rubio gets exasperated at the frequent suggestion that he is not putting in enough of an effort.

"Well, I just spent two hours meeting people and working here, and we're going to continue to do that," Rubio told reporters after a recent town hall in Muscatine. He added: "We love to be campaigning in Iowa."

Rubio's aides also argue that he is engaging with voters at a moment when they are beginning to pay close attention to the race. Rubio's campaign announced on Friday, for instance, that he will be spending three days in New Hampshire between now and Christmas, during which he will hold four town halls and a pancake breakfast.

Rubio's presidential campaign style is also in keeping with the way he has operated throughout his brief, meteoric career. Always impatient to make the next move up the ladder, he comes from a huge and diverse state where financial resources and a media presence are the most important ingredients of political success.

At the Muscatine town hall, Rubio spoke for 28 minutes before turning to questions. Just four people were recognized during the back-and-forth that lasted 17 minutes.

Rubio and his aides are adept at packaging and broadcasting the highlights of such events to supporters on social media. Rubio has one of the largest followings on Twitter among presidential candidates - more than a million people - and he uses Snapchat regularly.

"This is how I would reinvigorate American manufacturing," Rubio tweeted recently, linking to a page on his website featuring pictures and remarks from an Iowa speech.

The other Floridian in the race, former Gov. Jeb Bush, campaigns in a very different style, making himself more accessible to voters and the media than just about any other candidate. On Saturday, for instance, Bush held four town-hall meetings in New Hampshire. He tends to speak briefly, then open the floor to questions that can go on for an hour.

Yet, that has not paid off for him in the polls. Once expected to be the establishment front-runner, he has been lagging, while Rubio is somewhere among the top three candidates in most surveys.

Within that upper tier, Rubio's Iowa approach is in particularly sharp contrast with the breakneck pace being set by Sen. Ted Cruz of Texas, the rival with whom he has been having bitter public exchanges lately over immigration and national security.

"Nobody is going to win this presidential campaign by camping out in New York and D.C. and running a media campaign," Cruz told reporters recently when asked about Rubio. "There are some races that believe that their path to victory is courting the Washington establishment, is courting the big money donors all day long, as is hoping that their friends in the media can just push a narrative. Campaigns are about seeking the support of the voters."

At a candidate forum in Des Moines late last month sponsored by the Family Leader, a Christian conservative organization, Rubio spoke but did not stick around afterward, even though his campaign had rented a reception room. Cruz, on the other hand, was there until nearly midnight.

"We heard about it - and heard about it frequently - from Rubio supporters, that they were disappointed" at his abrupt exit, said Family Leader head Bob Vander Plaats, who endorsed Cruz less than three weeks after the forum.

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