Gallup polls the likely nominees:
The current results on Republicans' presidential nomination preferences suggest the 2012 contest could be more wide open than any since the winners began to be determined largely through state primaries and caucuses in 1972. Since that time, there has typically been a clear Republican front-runner before the nominating campaign got underway, including Richard Nixon in 1972, Gerald Ford in 1976, Ronald Reagan in 1980 and 1984, George H.W. Bush in 1988 and 1992, Bob Dole in 1996, George W. Bush in 2000 and 2004, and Rudy Giuliani in 2008. With the exception of Giuliani, the front-running candidate has prevailed.
Jonathan Bernstein recently made the same basic point.