Under criticism that he was not conservative enough, Bob Bennett, the three-term Republican Senator from Utah, lost his bid for nomination to a fourth term at a party convention Saturday.

So with Arlen Specter facing similar trouble here and Blanche Lincoln facing the same in Arkansas, it is setting up a rate of Senate turnover that hasn't been seen in 30 years. From Steve Kornacki at Salon:

To put this in some perspective, only four incumbent senators have lost primaries since the Reagan presidency: Joe Lieberman in Connecticut in 2006; Bob Smith in New Hampshire in 2002; Kansas' Sheila Frahm in 1996 (she was an appointed incumbent); and Alan Dixon in Illinois in 1992.

To find something similar to what we're seeing now, you have to go back to the politically turbulent late 1970s, when seven incumbent senators were defeated in primaries over the course of two election cycles. In the 1978 primary season, Democrats Paul Hatfield (Montana) and Maryon Pittman Allen (Alabama) and Republican Clifford Case (New Jersey) all lost in primaries. And in 1980, Democrats Richard Stone (Florida), Donald Stewart (Alabama) and Mike Gravel (Alaska) and Republican Jacob Javits (New York) all suffered the same fate.

It's worth noting that two of these senators, Hatfield and Allen, were appointed incumbents and had only been on the job for a few months at the time of their defeats. But besides Colorado's Bennet, all of the incumbents now facing defeat are well-established political commodities.