WASHINGTON (AP) — Mitt Romney may lead in delegates and Rick Santorum might have momentum, but neither of the two leading Republican presidential candidates is having an easy time exciting even his own voters.

Out of a dozen states where voters in the GOP contest have been polled, most Romney voters have said they strongly favor him in just five of them. A majority of Santorum voters felt that committed to him only four times out of 11 states where he was on the ballot and voters were surveyed.

Each man is struggling to consistently spark the intensity that could separate him from the pack.

Consider that Arizona is the only state where Romney had a higher proportion of voters expressing strong feelings about him than his rivals did. And Santorum hasn't had that edge in any state yet, despite an animated campaign style and passion for hot-button social issues like contraception that have contrasted with Romney's stiffer, more analytic manner.

On average, 50 percent of Romney voters and 49 percent of Santorum's say they strongly favor their candidate, with the rest expressing reservations about their man or a greater dislike for his rivals, according to entrance and exit polls of voters in 12 states.

When it comes to winning fervent devotion from his own supporters, both men trail former House Speaker Newt Gingrich and Texas Rep. Ron Paul. Those two GOP presidential campaigns are lagging but 58 percent of Gingrich's backers and 53 percent of Paul's, on average, say they strongly favor their candidate. Analysts say higher percentages of devoted supporters are common with candidates whose lesser backing leaves them with only the most committed followers.

Republican operatives express concern about Romney and Santorum. They say the figures raise questions about how quickly the GOP will be able to end its drawn-out slugfest and begin generating voter enthusiasm for a nominee to challenge President Barack Obama in November.

"At this point in the game, you should be drawing the troops toward you. The intensity about you should be pretty strong," said Mike McKenna, a Republican consultant not working for any of the contenders.

Political professionals generally consider intensity of support a key ingredient to a winning campaign. Such enthusiasm can help pave the way for everything from yard signs to campaign contributions to voters who show up on Election Day.

"Motivation is the name of the game in trying to get people to the polls," said Josh Putnam, a political scientist at Davidson University. "It makes campaigns' work easier if they can bank some votes, so to speak."

Though Romney and Santorum attract similar amounts of passion, there are different reasons why many of their voters are tepid, the polls show. For Romney, the chief explanation seems to be doubts about his empathy.

Among Romney's staunchest supporters, 80 percent say the former Massachusetts governor and Bain Capital executive understands average Americans better than the other candidates, the combined surveys show. That figure falls to just 44 percent among those who support him more tenuously.

Opponents of the wealthy Romney have at times derided him as being out of touch with typical voters, an image fed by remarks like saying his wife owns two Cadillacs.

Questions about Romney's credentials as a conservative also seem a liability. While hardly any of his most committed voters say he's not conservative enough, around 2 in 10 of his less passionate voters say so.

For Santorum, his intensity of support falls off among voters who are less religious and more moderate.

While 63 percent of his strong backers say sharing religious beliefs with a candidate is very important, just 35 percent of his less loyal voters say so, the combined polls show. Santorum voters who ardently back him are also significantly likelier than his less committed supporters to favor making abortion completely illegal, to be born-again or evangelical Christians, or to be tea party backers.

"I also have conservative values," said Patricia Moran, 71, of Pensacola, Fla., who said in an Associated Press-GfK poll last month that she backs Santorum with qualms. "But his whole women's issues and contraceptives thing has diminished my enthusiasm for him."

Santorum has said contraceptives conflict with his Catholic beliefs and states should be free to ban them.

Romney campaign pollster Neil Newhouse says that while Romney voters may not all exhibit fervor, they are attracted by more important qualities.

"His ability and focus on fixing the economy and his perceived ability to beat Barack Obama really overwhelm everything else in the data," Newhouse said.

Hogan Gidley, Santorum's communications director, said he didn't believe the polling data and contrasted its findings with the excitement he said he sees at the candidate's campaign events.

"Even if I believed that, what does that say about Mitt Romney," who has had the advantages of running for president longer and far outspending his rivals, Gidley said. "It says a whole lot more about Mitt Romney's supporters than ours."

Some Republicans expressed surprise that Santorum's support wasn't more passionate than Romney's, attributing it in part to the lesser familiarity that many voters have with him.

Several said it probably also reflected a desire by many Republicans to find a contender who can oust Obama. In the exit polls, Romney dominates among voters seeking that quality in a candidate.

"Santorum has become the vehicle to register your rejection of Romney," said Matt Mackowiak, a GOP consultant who had worked for one-time GOP presidential candidate Rick Perry. "But a lot of folks don't think necessarily that Santorum can either win the nomination or beat Obama."

Romney supporters say their candidate's focus on the economy accounts for the tempered enthusiasm of his supporters.

"He's not the guy who uses red hot rhetoric," said Sen. Robert Portman, R-Ohio, who has campaigned with Romney. "He's a thoughtful conservative who wants to roll up his sleeves and fix our problems. That may lead to less enthusiasm in some of the primaries, but I think it will serve him really well in the general" election.

The data comes from surveys of voters conducted for The Associated Press and the television networks by Edison Research in 12 states that have held GOP primaries or caucuses: Alabama, Arizona, Georgia, Iowa, Massachusetts, Michigan, Mississippi, Ohio, Oklahoma, Tennessee, Vermont and Virginia.

Exit and entrance polls in those states included interviews with 13,705 Republican primary voters.