Jonathan Martin (a.k.a. "J-Mart") has a smart post on that ABC/Washington Post poll showing Huckabee within the margin of error of Romney in Iowa. See in particular his thoughts on the challenge this poses for Romney:
To do so, however, they'll [the Romney campaign] need to do a few things. First they need to redefine Huckabee. He's gotten to where he is based on his charm and pluck as well as his social conservative and religious credentials. Romney will have to make the race more about his rival's platform and record, especially his less than orthodox views on immigration. Doing so won't be easy, given Romney's own flip-flop skeletons, but it will be doubly so given that Iowans typically reject overt negativity. Team Mitt will likely be aided by the Huck-hating Club for Growth in trying to take down the Arkansan, but those third-party attacks could also only stand to engender sympathy for Huckabee.
Actually, I think it's going to be triply difficult, given the apparent intensity of Huckabee's support. According to the Post's write-up of the poll:
[A]lmost half of Huckabee's supporters (48 percent) said they would definitely vote for him in January and only a quarter said there was a good chance that they would change their minds before the caucuses. In contrast, just 29 percent of Romney's backers said they would definitely vote for him, while 42 percent said there was a good chance that they could vote for someone else at the caucuses.
Combine this development--high intensity for Huckabee, lower intensity for Romney--with the distaste for negative campaigning J-Mart cites, and I think Romney really has to thread a needle here. Any attack is as likely to rally Huckabee's true-believers and turn off Romney's tepid supporters as it is to give the tepid Huckabee supporters second thoughts.
If I were Romney, the one thing I'd consider before launching an attack is how high Huckabee's ceiling is in Iowa. (I'm sure team Romney is having that conversation as I write...) I don't know the answer to that, but there's a case to be made that he's pretty close to it now--it's still very hard to see him winning the nomination, after all--in which case maybe you hold off, concede Huckabee his bloc, and just concentrate on driving up your own turnout and winning over current undecideds. Also, as J-Mart points out, Fred Thompson is competing with Huckabee in Iowa for a similar pool of votes. If Romney stands down, there's a pretty good chance Thompson will do his dirty work for him. (Conversely, Thompson is probably hoping Romney hits Huckabee so that he can scoop up all the voters turned off by the exercise.)