Bryan Caplan links a 2007 study that looks at voter turnout and weather, and specifically tests the conventional wisdom that rain helps Republicans (by disproportionately surpressing the Democratic vote).
The findings appear to be that bad weather does help Republicans and does supress turnout. However, in studying presidential elections, he finds few that would have had their outcome changed. Here, however, was one exception:
The results of the zero precipitation scenarios reveal only two instances in which a perfectly dry election day would have changed an Electoral College outcome. Dry elections would have led Bill Clinton to win North Carolina in 1992 and Al Gore to win Florida in 2000. This latter change in the allocation of Florida's electors would have swung the incredibly close 2000 election in Gore's favor.
Since we know from Gore that heavy snow, no snow, heavy rain, and no rain are all caused by global warming, his 2000 electoral defeat was obviously caused by manmade CO2.