Washington (CNN) -- Herman Cain may be battling with Mitt Romney atop the Republican presidential polls, but Romney remained the "target du jour" Sunday for both his current and possible future competition.
Two other rivals for the GOP nomination -- Texas Gov. Rick Perry and a fellow Texan, Rep. Ron Paul -- as well as David Plouffe, a senior adviser to President Barack Obama, all accused the former Massachusetts governor of flip-flopping on major issues in a bid to strengthen his conservative credentials.
The targeting of Romney on Sunday talk shows demonstrated how he remains the most formidable contender in the eyes of his rivals, even with Cain's rise to top-tier status in the Republican race.
A new Des Moines Register poll in Iowa showed 23% of likely caucus-goers backed Cain while 22% supported Romney, making them the frontrunners for now in the state that will give the first formal read of the race on January 3. Paul was the only other candidate in double figures at 12%.
Plouffe, offered a chance to size up Cain on NBC's "Meet the Press," gave a minimalist response that the former pizza executive seemed "to have tapped into something."
Without prompting, he was much more detailed and pointed about Romney.
"He has no core," Plouffe said, citing issues such as climate change, gay rights and abortion on which he accused Romney of shifting to the right politically to try to satisfy conservatives.
"You get the sense with Mitt Romney that, you know, if he thought it was good to say the sky was green and the grass was blue to win an election, he'd say it," Plouffe added.
Romney's team fired back quickly, issuing a statement by spokesman Ryan Williams that accused Obama and his aides of trying to divert attention from their own shortcomings.
"President Obama can't run on his abysmal economic record and he is desperate to distract from the historic loss of middle-class jobs that has occurred on his watch," Williams' statement said. "Americans won't be fooled by false and negative attacks."
Perry, appearing on "Fox News Sunday," touted his own pedigree as a self-described "consistent conservative" compared to Romney on issues such as gun control and abortion.
"We are very, very different from the standpoint of consistency on those issues I've just mentioned," said Perry, who was tied for fifth in the Iowa poll with 7% support.
Paul told CNN's "State of the Union" that Romney has been "pretty smooth" in responding to criticism over changing his position on issues. He made a point of noting that, unlike Romney, "they haven't challenged me for flip-flopping, so I'm very proud of that."
Cain, meanwhile, spent much of his appearance on the CBS program "Face the Nation" clarifying past statements or responding to criticism, rather than launching zingers against Romney or other rivals.
He distanced himself from a previous comment that abortion in cases of rape and incest should be up to the family, declaring: "I am pro-life from conception, period."
Last week, Cain said on CNN's "Piers Morgan Tonight" that he was against abortion rights, but when asked about rape or incest, Cain said the government shouldn't have a say on those issues and the decision should be left to the family.
Some social conservatives slammed Cain for his remarks in the following days, prompting him to be vocal about his "pro-life" stance in subsequent campaign appearances and interviews.
Asked Sunday about his plan to set flat 9% tax rates on corporate and personal income while also assessing a 9% national sales tax, Cain denied it would mean a heavier tax burden for low- and middle-income Americans.
Setting a flat corporate tax rate would eliminate what Cain called "embedded" taxes in the production process, bringing down consumer prices to result in overall savings benefits, he said. Embedded taxes are estimated by some economists to comprise 30% or more of a product's final cost, he said.
In addition, Cain yielded to prodding by "Face the Nation" host Bob Schieffer to publicly declare his opposition to smoking in the aftermath of a novel campaign ad in which Cain's chief of staff takes a drag on a cigarette and blows smoke at the screen.
"Young people of America, all people, do not smoke," Cain said to the camera after Schieffer, a former smoker and cancer survivor who acknowledged he was editorializing, admonished the ad as possibly giving a pro-smoking message. "It is hazardous and it's dangerous to your health. Don't smoke. I've never smoked and I have encouraged people not to smoke."
To further prompting from Schieffer, Cain added: "It is not a cool thing to do."
CNN's Gabriella Schwarz contributed to this report.