The Dems lost most of America in '68 when the radical leftists decided that the mainstream candidates weren't far enough to the left. The riots that ensued were televised, and a lot of people were turned off by the display of pure anti-Americanism.
Expect to see more of the same this summer at the two conventions. With the same results.
There is no way that the Left can unmake the bed they are now lying in. They are in with hard-core stalinists and other lunatic fringe folks who will be much more visible in the demonstrations sure to come. For every placard carried calling for the US to pull out of Iraq, there will be several calling for support to the Iraqi terrorists who are killing US soldiers.
For every inocuous Uncle Sam stilt walker, there will be several venomous, swearing trotskyites.
And for every vote these demonstrations swing towards the Left, they will cause a backlash of five to the Right.