...I'm not sure I agree with the premise that an endorsement from Mr Perry before the primary will help Mr Gingrich win South Carolina, or at least solidify his advantage there over Rick Santorum. If the issue is that the opposition to Mr Romney is currently split--between three candidates, if you're conceiving of it as the socially conservative vote, or between four, if you see it as anti-Romney--it's not clear to me that Mr Perry's withdrawal would send his voters to the Gingrich camp, even if he did endorse Mr Gingrich. Such an effect may be discernable when the candidate who withdraws is a close substitute for a candidate who remains, but the effect probably has more to do with the similarities between the candidates in question than with the endorsement. In this case, Mr Perry isn't an obvious analogue to Mr Gingrich. The voters who are drawn to him because he's advocating the limited-government line might move on to Mr Gingrich, but those who are drawn to his religious rhetoric would be more likely to switch to Mr Santorum. So Mr Perry's withdrawal wouldn't necessarily help Mr Gingrich in the way that Mr Erickson suggests, and it might even hurt him, if Mr Santorum ends up with a higher share of the votes. We should note, too, that it's not at all clear Mr Gingrich is the strongest candidate to beat Mr Romney. He had a cracking debate performance on Monday, and he is polling above Mr Santorum in South Carolina, but Mr Santorum crushed him in Iowa, and the race has been mutable.
I should add here that some of Mr Perry's supporters (not that he had that many) will no doubt go to Mr Romney; as Nate Silver points out, many people who favour another candidate nonetheless think Mr Romney is acceptable.