But there's been a difference in the effectiveness of the attacks against various candidates. In April, when Donald Trump was doing well in some national polls, he was quickly cut down by attacks against his authenticity. When Michele Bachmann entered the race and began doing well in Iowa, she was swiftly humbled by attacks against her electability. Bachmann's fall allowed her to be overtaken by Perry. Now Perry has been on the receiving end of attacks against his electability and conservatism. It's likely he'll fall from his comfortable frontrunner position in the polls.
Romney, too, suffered a fall in the polls. The shallow depth of Romney's support was seen when Perry was able to easily replace him as the leader in the polls. There's been a contrast, however. Prior to Perry's entry, Romney's poll numbers were not worn down by three months of attacks against him. Also, unlike other candidates who have gone down and stayed down, Romney is going back up. That's most clearly seen in Mitt Romney's Intrade odds, and it's likely to be seen in future polls.
Why the difference? Romney's main weakness has always been Romneycare. He erred in failing to repudiate it, and debate moderators have asked him about it in all four debates. The reason Romney has not been hurt more by Romneycare is that he is far better prepared to defend himself than other candidates are prepared to attack him. Romney's answers are well-rehearsed. His opponents' attacks are not. The exchanges on Romneycare have all followed some variation of the following pattern:
1. Debate moderator asks Romney about Romneycare.
2. Romney defends himself and reiterates his commitment to repealing Obamacare.
3. Opponent says that Obamacare was modeled on Romneycare.
4. Romney refutes this by listing differences between Romneycare and Obamacare.
5. Opponent stammers, "Come on, we know they're the same."
To defeat Romney, opponents need to have as substantive and well-rehearsed counter-points for step #5 as Romney has for steps #2 and #4. The same could be said for other attacks aimed against Romney, e.g. criticisms of his work for Bain Capital, where Romney either created jobs or fired people, depending upon the source.
Another reason Romney's position appears to be impregnable is the fact that he's running in New Hampshire, rather than Iowa. In Iowa, many candidates are staking the lives of their campaigns. Bachmann, Rick Santorum, Herman Cain, and (unwisely) Newt Gingrich all see themselves in a do-or-die situation in Iowa. Perry is competing there as if he needs to win, though he's likely to win South Carolina even if he loses Iowa. In New Hampshire, Romney is basically by himself. Only Huntsman is targeting New Hampshire, and he's hardly in the race.
Thus, the configuration of the early primaries has been to Romney's advantage: The other candidates have less incentive to attack him, as their top priority is defeating competitors in Iowa. Until the Iowa Caucus, Romney will not be the focus of their attacks. And that's a long time from now.