Sure, we see the angle: Gingrich excels in debates and he knows it, and in light of his threat to Romney in Iowa, his participation all but dares the yet-uncommitted Mitt to irk the pro-Trump rump of GOP voters by refusing. As a serious contender running a campaign with maximal pride in its own seriousness, Gingrich lowers himself by association with this consummately unserious man. Romney should refuse to follow suit.

Weigel has more on Trump's motivations. Patrick Caldwell wonders if Trump will revive birtherism: