Bush is the latest in a string of Republican former governors who have backed Thompson. He also has the support of former Arkansas Gov. Mike Huckabee (R), former Pennsylvania Gov. Tom Ridge (R), former Idaho Gov. Dirk Kempthorne (R), former Utah Gov. Mike Leavitt (R), former Oklahoma Gov. Frank Keating (R), former Kansas Gov. Bill Graves (R), former Massachusetts Gov. Bill Weld (R) and former Michigan Gov. John Engler (R).

Neumann has the backing of some Tea Party groups as well as the deep-pocketed, fiscally conservative Club for Growth, while Hovde is a self-funder who is spending heavily on the race.