DES MOINES -- In an interview today, Gov. Mitt Romney said he authorized his campaign to air his new ad, which takes Gov. Mike Huckabee to task on immigration, because he believed that Huckabee's sudden rise hasn't been accompanied by the type of scrutiny generally given to top-tier presidential candidates.

While national media outlets have begun to scrutinize Huckabee's record in earnest, Romney said he saw much less of that sort of coverage in Iowa.

And Romney hinted that immigration is only the start of his comparisons. On taxes, pardons and commutations, he said Huckabee's record compares unfavorably with his.

I spoke with Romney at the Marriott in downtown Des Moines a few minutes before he headed into a prep session for tomorrow's debate. I called the ad a "contrast" ad, not a "negative" ad -- trying to preserve the distinction between the two -- and Romney seemed to much prefer that characterization.

The full interview / podcast will be posted later.

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