For more than six months, worried conservative chieftains talked up the need to unite behind a single rightist candidate in order to block the potential victory of the “mushy moderate” from Massachusetts, Mitt Romney. Now, on the eve of crucial primaries in Michigan and Arizona, and with Super Tuesday looming just one week later, some of those same leaders speak privately of the need to unite behind that same, once-dreaded Romney in order to avert an even more dire disaster: the nomination of Rick Santorum.