There has been much speculation over who Thompson would hurt most by entering the GOP nomination race. However, that's all it's been--speculation. However, a look at CNN's polling numbers for the first five states reveals an interesting pattern:

Iowa
25%--Romney
17%--Thompson
15%--Guiliani
7%--McCain

Nevada
25%--Thompson
20%--Romney
17%--Guiliani
8%--McCain

New Hampshire
28%--Romney
20%--McCain
20%--Guiliani
11%--Thompson

South Carolina
25%--Thompson
21%--Guiliani
11%--Romney
7%--McCain

Florida
27%--Guiliani
21%--Thompson
13%--McCain
7%--Gingrich
6%--Romney

When Thompson is up in the polls, McCain is down. Added together their poll percentages range from 24-34%. By contrast, adding Thompson's numbers to Romney or Rudy give ranges from 27-45% and 32-48% respectively.

In addition, there is no pattern--sometimes Romney or Rudy is up and Thompson down (and vice versa), and sometimes Romney or Rudy and Thompson are both up. On the other hand, McCain and Thompson never finish 1-2 in a poll, and never are both at 15% or higher in the same poll.

While I don't claim to be a statistician, this is the only pattern I can see emerging from a combined look of these five first-state polls, and it demonstrates that Thompson is takest the biggest bite out of McCain's numbers.