If it’s all about Huckabee’s rise, as some have contended, then by all means go after Huckabee’s substantive record. So say Ross Douthat (with good links) and James Poulos (also with good links). (Douthat also has more here and here.) Romney’s problem in the Republican electorate is with evangelicals (see this Pew report for the details). He can’t persuade them that he’s a Christian, but he can try to persuade them that his Mormonism shouldn’t matter. The question then would be whether his substantive positions, character, and electability are superior to Huckabee’s (leaving aside for a moment the other candidates). This is, of course, where the debate ought to be.
Update: Count Jonah G. in this camp.