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Six days before the Iowa caucuses, Republican presidential candidates revved up the offensive as new polls showed Mitt Romney once again leading the pack and Newt Gingrich dropping sharply in popularity.

Twenty-five percent of likely GOP voters in a new CNN/Time/ORC International Poll picked the former Massachusetts governor as the candidate of choice. Ron Paul's support jumped to 22 percent while Rick Santorum more than tripled his support since early December to 16 percent. Gingrich, who topped the poll earlier this month with 33 percent of approval, plunged to 14 percent, below Santorum.

The poll shows a similar picture in New Hampshire, which will hold the nation's first primary a week after the Iowa caucuses.

Romney leads in that state with 44 percent of the vote, followed by Paul at 17 percent and Gingrich at 16 percent.

Gingrich's peak has eroded amid a scathing campaign by his rivals, particularly Romney, painting the former House speaker as a flip-flopper and a Washington insider. Romney has even questioned Gingrich's personal history - his two divorces, three marriages and two religious conversions - in a not-so-subtle campaign ad.

Nationally, Gingrich's lead over Romney has eroded quickly and the two are now tied for first place in Gallup's latest tracking poll. The numbers are a sharp contrast to earlier this month when Gingrich enjoyed a 15 percentage point lead over Romney.

The attacks between campaigns continued in full force today, with most candidates focusing on  Paul, the unlikely candidate who has emerged as the surprising front-runner in a heated race.

Romney became the latest in line today to take a swipe at the Texas congressman, a day after Gingrich unabashedly said that he would not vote for Paul should he win the GOP nomination.

Romney today indirectly blasted Paul's comments made earlier this month that cautioned against "jumping the gun" on Iran.

"At the same time the greatest threat Israel faces and, frankly, the greatest threat that the world faces is a nuclear Iran," the former Massachusetts governor said during a meet-and-greet at a coffee shop in Muscatine, Iowa. "We have differing views on this. Some of the people - actually one of the people running for president - thinks it's OK for Iran to have a nuclear weapon. I don't."

Michele Bachmann, who is hanging at the bottom of the polls, said Paul would be a "dangerous president" and he would "wait until one our cities is wiped off the map" before confronting Iran.

As he was attacked on his congressional record and national security policies from all fronts, Paul unleashed his own line of attack today. In a new ad to air in Iowa and New Hampshire, Paul blasted Gingrich and Romney as "serial hypocrites" and "flip-floppers" that "can't clean up the mess." The ad touts the longtime congressman as "consistent" and "incorruptible" in a clear rebuke to his rivals' flip-flopping statements on the individual mandate in the health care law.

Paul, however, ignored questions by reporters about controversial newsletters that he published in the late 1980s and 1990s containing racist, anti-Semitic and anti-gay screeds.

This is not Paul's first foray in the race to be the Republican candidate for president - he also ran in 2008 - but his libertarian-leaning views have attracted much more attention this time around. He's been particularly helped by the baggage that leading rivals Romney and Gingrich carry.

Romney continues to be haunted by the ghosts of the Massachusetts' health care bill, which was the model for the Democrats' and President Obama's national health care law.

Though Gingrich once lavished praised on Romney, the Massachusetts plan and the individual mandate so staunchly opposed by conservatives, he admitted today he was wrong and pushed his rival to do the same.

Gingrich said he and the plan's other conservative supporters didn't realize that it was using "taxpayer money to pay for abortions" and  "that they had accepted Planned Parenthood by law as one of the members of their health board."

"I've said in the debates, yes, I used to be for mandates. I was wrong. Why doesn't Mitt admit it? He's still for the mandate in Massachusetts. It doesn't work, It's going to bankrupt the state," Gingrich said. "The difference between Romney and me is that we both used to have the wrong idea. I'm willing to say it was the wrong idea. He's not."

"I used to say I was for mandates and was wrong," Gingrich continued. "Now he won't admit he's wrong."

Despite the attacks on him, Romney continues to hold the upper hand when it comes to important endorsements. Today, the former senator from North Carolina, Elizabeth Dole, endorsed Romney, as did the Iowa City Press-Citizen.

Gingrich also continues to be dogged by his political baggage. He has staunchly denied getting paid to lobby after his departure from Congress, but new claims from his peers refuted that claim. Two Republican congressman told the Des Moines Register that Gingrich pushed GOP congressman who were on the fence about the Medicare expansion bill in 2003 to vote for the controversial prescription-drug program.

"He told us, 'If you can't pass this bill, you don't deserve to govern as Republicans,' " said Rep. Jeff Flake, R-Ariz. "If that's not lobbying, I don't know what is."

Former Rep. Butch Otter, who is now the governor of Idaho, confirmed that claim, adding, "I can't define lobbying, but as a Supreme Court justice once said about pornography, I know it when I see it. I felt we were being lobbied."

Both have endorsed Romney.

Other candidates are now hanging by a thread in Iowa, where the three-way competition has left little support to go around. Texas Gov. Rick Perry is especially trying desperately to get back in the game. He and his supporters have spent more money in Iowa on TV ads then all the other candidates combined. But that has yet to translate into a jump in polls.

Gingrich's campaign, however, is touting one unlikely honor. The former congressman from Georgia ranks sixth on USA Today/Gallup's list of the men most admired in 2011, well below President Obama but on par with the pope and Donald Trump. Another Republican candidate also made that list. Minnesota Rep. Bachmann also placed sixth on the list of most admired women, in line with Queen Elizabeth II and Bachmann's  own idol, Margaret Thatcher.

ABC News' Karen Travers contributed to this report.