Associated Press
Republican presidential candidate Texas Gov. Rick Perry sits atop polls for the GOP presidential nomination race.

WASHINGTON — Rick Perry's Republican rivals are struggling to find a coherent, easy-to-grasp argument against the Texas governor, who tops GOP presidential polls despite attacks from all sides.

In fact, it's the "all sides" nature that complicates the opposition's message. Republican voters who watched last week's presidential debate and its aftermath might wonder: Should I see Perry as too conservative or too moderate?

Perry is the newest face in the GOP race and his opponents are determined to define him for primary voters, casting him as liberal, conservative and unelectable. They hope their characterizations of the front-runner take hold before he has a chance to sway opinions.

Mitt Romney depicts Perry's criticisms of Social Security as too far to the right. "If we nominate someone who the Democrats could correctly characterize as being against Social Security, we will be obliterated as a party," the former Massachusetts governor said recently.

On immigration, however, Romney and other opponents say Perry veers too far left. The governor opposes a fence along the entire border with Mexico, and he granted in-state college tuition to illegal immigrants in Texas.

Meanwhile, Minnesota Rep. Michele Bachmann leads a chorus that contends that Perry is too lax about individual freedoms because he wanted Texas to vaccinate all schoolgirls against a sexually transmitted disease.

This anti-Perry strategy forces voters to sort through subtleties and contradictory narratives. Fair or not, it's easier for people to grasp bold, unambiguous images of politicians as conservative or liberal, strong or weak, and so on.

Overall, Perry's record is mostly conservative. But he's parted ways with his GOP base on a handful of issues, including immigration and the vaccine for the human papillomavirus.

Perry's rivals will get more chances to probe for political soft spots this week, in a series of forums in Florida and Michigan. On Thursday, Republicans candidates gather for another televised debate.

For now, their tactic is to "criticize Perry on Social Security from one angle, and on immigration from the other," said Dan Schnur, a University of Southern California political scientist and veteran of several Republican campaigns.

Terry Holt, a Washington-based GOP strategist, said Perry continues to do well because his opponents' criticisms are missing a broader point while barely denting his main strengths: His image as a bold, honest, can-do leader.

"It's a bit too tactical, and it ignores the larger imperative: Can you be an alternative to the vision Barack Obama offers? Can you be authentic?" Holt said.

Rich Galen, another veteran GOP campaign strategist, said the real goal of Perry's rivals is to convince enough Republican activists — including those who like Perry — that he can't defeat Obama.

"What they're trying to do, really, is not influence Republican primary voters directly," Galen said. Instead, they want to convince "independents and moderates that Perry is not trustworthy or is too kooky."

If die-hard conservatives believe crucial independent voters would reject Perry in November 2012, Galen said, they may turn to Romney or others, even if they like Perry's positions. "It's a bank shot," he said.

Some Republican insiders question the strategy of trying to turn conservatives against Perry with the "he can't beat Obama" claim.

"I don't think they can make that case," said Rep. Jack Kingston, R-Ga., a 19-year House member. Perry has a good staff, strong fundraising skills and "a good story on jobs" as governor, said Kingston.

Schnur agreed. "Arguing electability is usually a loser in the presidential primary," he said. "Ask Hillary Clinton."

A recent Washington Post-ABC News poll of Republicans put Perry in front as the candidate most likely to beat Obama. But a USA Today/Gallup poll suggests the latest hits from his rivals might have an impact. By more than 2-to-1, GOP voters said Perry's criticisms of Social Security would hurt rather than help his chances of being elected president.