The Fred Thompson campaign has a great deal of support because there is a great dissatisfaction with the current field of candidates.
According to the latest CBS News Poll taken during June 26-28, 2007 of 212 Republican primary voters, 61% want more choices. This suggests that there is a large number of dissatisfied voters and this 61% of voters can easily switch from candidate to candidate. Fred Thompson figures to get a lot of that support once he gets into the fray.
USAElectionPolls.com has a stat sheet for Fred Thompson polls.