Clinton (54%) vs. Obama (69%)

The betting site Intrade allows people to bet on politics. You can use the odds of various bets to determine peoples' estimates of different candidates winning various elections.

Bettors give Clinton a 48% chance of winning the Democratic nomination and a 26% chance of becoming President. This means bettors believe that if she wins the Democratic nomination she has a .26/.48 or a 54% chance of winning the general election.

In contrast, bettors believe that Obama has only a 30% chance of winning the Democratic nomination but a 20.7% chance of winning the general election. This means that bettors believe that if he wins the Democratic nomination he has a .207/.30 or a 69% chance of winning the general election.

If Democratic voters were rational they would take all this into account and view Obama's nomination far more favorably.

(Mankiw has previously done this type of calculation.)