I guess he finally had to since he doesn’t have any other targets:

For the past week, he has been loud and proud about sticking in this race until one candidate has the necessary delegates to take the nomination. With only Sen. John McCain, R-Ariz., between the governor and the stage in Minneapolis/St. Paul this summer, Huckabee, who has, so far, played nice when it comes to the Arizona senator, looks to be taking off the gloves.
In a press conference meant to tout his victory in Saturday’s Kansas caucuses, Huckabee was asked about an e-mail from McCain’s Virginia co-chair, Chris Saxman, regarding pro-Huckabee push polling in Virginia. “It is time for Mike Huckabee to accept responsibility for these despicable tactics,” wrote Saxman. “Take a stronger stand against them, and help unite the party, as many prominent GOP leaders have done.”
Huckabee shot back from his podium at the Washington, D.C., Marriott, “if anyone needs to take responsibility, it’s John McCain, because McCain Feingold Campaign Finance act creates that nonsense.” He later added, “So, if Sen. McCain hadn’t pushed that bill through, created a stifling of free speech for some groups and an opening for others, we wouldn’t be having this discussion today; so, for his people to call on me to stop it, has to be one of the most bizarre comments in this election cycle.”