That's what David Yepsin of the Des Moines Register concluded after speaking to GOP Senator Chuck Grassley and finding that he was unable to name anyone who was seriously thinking of taking on Senator Tom Harkin, his Democratic counterpart.

"Harkin’s already banked over $2 million," Yepsin writes. "Serious challengers get scared off by that, and by history. According to Harkin’s office, the Congressional Research Service has said the Iowa Democrat has defeated more sitting Republican members of Congress than any Democrat in the history of the Senate."

"So it’s little wonder top GOP office-holders aren’t interested in becoming the sixth."

Sounds pretty convincing to us.