Just my take -- but I believe the general feeling is that there are more registered Dems than Republicans. Since Republicans (generally) vote more dependably, any increase means most of the people turning out are Dems.
For what it's worth.
they like to ignore the fact that sKerry's supporters don't care about him much, and Bush's supporters consider themselves 'VERY' committed...
so if the weather stinks, the dems worry that their 'not very invested' supporters won't bother going out to vote...