Jessica and I were on a walk this morning and she made a brilliant point about the Democrats: their imminent nomination of Barack Obama is a tacit admission that they can't win a presidential election purely on the issues. They know that the left's socialist domestic agenda and internationalist foreign policy will only attract ~30% of American voters, so they're forced to adopt Obama over Hillary Clinton in the hope that his charisma will draw in the other 20% + 1 the Democrats need to win the White House.
On the other hand, Republicans are soundly rejecting more charismatic candidates in favor of John McCain. They actually believe (wrongly or rightly) that they can win the election based on the issues. Republicans believe that 50% + 1 of the electorate prefer their policies or can be convinced, and that they don't need a pretty man in an empty suit to do the selling. Maybe Republicans are wrong and America won't agree with them on the issues, but the nomination of John McCain at least proves that the Republicans are sincere.