In Sunday’s GOP presidential debate, Senator Sam Brownback went hard after former Massachusetts Governor Mitt Romney for changing his position on abortion. Governor Romney explained his recent shift to a pro-life stance, adding: “I get tired of people that are holier than thou because they’ve been pro-life longer than I have.” This exchange raises a larger question: in what instances should changing a position on issues be held against a candidate? When does it help, and when does it hurt?

One way to answer is to examine the explanation for the change. Is it plausible and serious? Another has to do with the magnitude of the change. Are we talking about an evolution in views, or a dramatic alteration? Yet another factor has to do with timing. Did the change in position coincide with other (higher) political aspirations and the political calendar?

The other matter that comes into play is the reputation of the candidate. Ronald Reagan changed his views on abortion, and he even violated his own administration policy when he sold arms for hostages. Yet Reagan was seen as a man of deep and admirable convictions. He had been a warrior for conservatism, especially during its wilderness years. And so he had stored up a great reservoir of trust and good will, which he had to draw on from time to time.

Romney is in a different place. He is seen (with justification) as highly intelligent and competent. But he has not yet established himself as a person of unshakeable convictions. On abortion, Romney’s change has been especially dramatic. That’s fine; better he embrace a culture of life than distance himself from it. But when you combine this with his shifts on guns, gay rights, and (to some degree) immigration, you can’t help noticing that he has created a stress fracture that could turn into a clean break—which was exactly what Senator Brownback was trying to investigate during Sunday’s debate.

Mitt Romney is still in good shape. He’s done reasonably well in the debates, he’s raised a lot of money, and he’s in a strong position in both Iowa and New Hampshire. He is clearly trying to position himself as the conservative alternative to Rudy Giuliani (a position Romney will soon share with Fred Thompson). Many conservatives like Romney, but are a bit wary. A candidate’s shifting on issues is not unprecedented or disqualifying. It may even be understandable. But Romney has just about used up the number of times he is able to do so before permanently alienating his constituency.