WASHINGTON - US Republican presidential contender Jon Huntsman's campaign manager resigned on Thursday in a blow to an election effort that has failed to catch fire.

Susie Wiles told the Miami Herald the campaign job was "an overwhelming commitment" and it was time for her to move on.

Huntsman, a former Utah governor who left his post as ambassador to China earlier this year, has lagged in polls since entering the Republican race for the right to challenge President Barack Obama last month.

But Wiles and campaign officials said the move did not represent a shake-up.

"I signed up to get it started. It's like a phase. This morning I said it's time to move on," Wiles told the Herald.

Huntsman launched his campaign last month but has failed to win much traction in opinion polls, lingering in the low single digits. He took in about $4.1 million in contributions in the second quarter of the year, although nearly half of that was from his own pocket.

That put him on par with rivals like US Representative Michele Bachmann and former Minnesota Governor Tim Pawlenty but well behind front-runner Mitt Romney's $18.4 million haul.

Senior Huntsman adviser John Weaver said the campaign was shifting into "phase two" of the 2012 race.

"In just under three months Governor Huntsman has returned from China, launched a campaign and created a strong infrastructure in the three early primary states," he said.

Communications director Matt David will take over as campaign manager, a campaign official said.

Switching managers early in a campaign is never a good sign, although it is hardly fatal. Both 2008 Republican nominee John McCain and 2004 Democratic nominee John Kerry shifted their senior campaign aides before capturing the nominations.

Huntsman is the second Republican 2012 candidate to experience a change at the top this year. Former US House of Representatives Speaker Newt Gingrich suffered a mass walkout of senior staff last month.