UPDATE: (Tuesday 22 Feb): Hooray! I was wrong! Fred drops out!
There is a lot of speculation that Fred Thompson will drop out of the race in the next couple of days, while this certainly would not surprise me (and he really should drop out), I think he may just stay in the race. The reason for this is because of John McCain.
Here are some premises from which I will build my case:
1. McCain (and Huck, but this is irrelevant) absolutely despises Mitt Romney.
2. McCain and Thompson are friends and have worked closely together in the Senate.
3. It is likely that Fred would endorse McCain if he got out of the race.
4. (This is key) Despite that endorsement, a majority of Fred supporters would and are flocking to Mitt.
Considering this, I would not be the least bit surprised to find out that the McCain camp approached the Thompson camp and asked them to stay in the race to siphon off conservative votes from Mitt. Currently, Fred is averaging about 8-10% in Florida. If Fred were to get out and endorse McCain, I would suspect that only about 2% would go to McCain, 2% would go to Huckabee, and the remaining 4-6% would go to Mitt. So for McCain that is a net loss of 2-4% to Mitt. Thus, it is much better for McCain to keep Thompson in the race as long as possible, even despite a potential big endorsement.
Let me add for good measure that Huckabee will attempt the same thing and will not drop out after Super-Tuesday, despite him knowing full well that he cannot win. It will all be about getting McCain in the White House and keeping Mitt out of it.