flip-flopper

Secondly, he will rightly be labeled as a flip-flopper on this issue for his “evolution” on it in just two years.

But on the other hand, if he does that, he's a flip-flopper.

In politics, the flip-flopper label is deemed deadly, the fingering of a candidate with no fixed principles.

Why, then, has the flip-flopper label attached itself so much more firmly to Romney than to the equally flexible Gingrich?

He was being defined as a flip-flopping Mormon—or a Mormon flip-flopper.