Mitt Romney's three-state shellacking by Rick Santorum laid bare the Republican White House frontrunner's Achilles heel -- an inability to motivate the party's conservative base.

The enthusiasm gap -- revealed in low overall turnout at caucuses and primaries and Romney's tepid polling numbers -- could end up handing President Barack Obama a second term should it continue through the November election.

"Certainly those conservative evangelicals are not going to vote in droves for Obama, but what they could do -- and what they did in 2008 -- is that they could stay home," said Michael McDonald, a voter turnout expert at George Mason University.

Another pitfall for Romney is low turnout among the moderate and independent voters who often decide US elections, even as the Republican primaries have attracted huge amounts of media attention, McDonald said.

"If he's not energizing and exciting those people right now, there's the danger that he will lose them to Barack Obama," he told AFP.

Low turnout at the polls was mirrored in a survey released Wednesday which found that only 54 percent of Republicans are "very excited" about voting in November's presidential election, compared with 58 percent of Democrats.

The Public Policy Polling survey also found that 25 percent of conservatives are "not at all excited" to vote in November, compared to only 16 percent of liberals. Even among ultraconservative Tea Party voters, excitement has dropped nine points from last July to 62 percent.

"Obviously all this could change once Republicans are settled on a nominee," Public Policy Polling said of the results, but for now Obama's prospects "are looking the best they have in quite a long time."

Nearly every one of the more than two dozen voters approached at rallies and caucuses in Minnesota this week told AFP they would vote for any Republican on the ballot in order to unseat Obama.

But several expressed serious reservations about Romney, a multimillionaire former governor of left-leaning Massachusetts, who is struggling to prove his conservative credentials and fight off accusations of being both a "flip-flopper" and out of touch with working-class Americans.

"Four years ago I held my nose and voted for (John) McCain, but I consider Romney to be even more liberal than McCain because of his voting record," said Eric Evander, 51, a computer technician and Santorum supporter.

"I would have a very hard time (voting for Romney.) I can't say yes and I can't say no, but I hope it doesn't come to that."

Melanie Kocon, a homeschooling mom, said she would stay home rather than vote for Romney or Newt Gingrich because she doesn't trust them and is disgusted by their mudslinging on the campaign trail.

"I realize that might mean Obama would be reelected, but I don't think Minnesota is the be-all and end-all of this race," Kocon said after a Santorum rally in Waconia Sunday.

Deb Nelson, 51, another home schooling mom, said that while she'd vote for Romney she "won't really campaign for him."

It would be different if Santorum -- a "true conservative" -- got the nomination, she said.

"I got a dozen people here today," she said at a Santorum rally in suburban Minneapolis on Tuesday. "This just charges me up. That kind of stuff may not happen for Mitt."

Romney has the best-funded and most organized campaign, as well as the backing of the Republican establishment, which believes he stands the best chance of winning over moderate and independent voters.

But the endorsement of former Minnesota Governor Tim Pawlenty did little to help Romney win the midwestern state.

Santorum captured 45 percent of the vote while Romney won only 17 percent, landing him in third place behind Texas congressman Ron Paul.

Perhaps most troubling for Republicans was the turnout.

Less than 26,000 people attended Republican caucuses in Minnesota on Tuesday, down from nearly 63,000 in 2008, when Romney beat eventual nominee John McCain after capturing the support of 41 percent of the vote.

Winning an election takes more than just the money to hire local staffers and pay for ads, turnout expert McDonald said.

"You have to have people who believe in what they're saying, going door to door and talking to people and convincing them," he said.

"You need those volunteers, you need to excite the base. Romney needs to figure out how to do that. Because the Obama campaign is already doing that."