Dems and Whigs: I don't place too much stock in the parallel. First, slavery was a major factor in dividing and destroying the Whigs. I don't see a similar divide on any issue for the Dems. Of course, the Dems do have to worry that minorities will one day desert the party. Were it not for the fact that blacks and hispanics (aside from Cubans) vote overwhelmingly for Democrats, the party would be a footnote after 1980. (And this is, of course, why Al Sharpton gets to go to all these debates -- even though he's a disreputable shakedown artist and race hustler -- and have soi disant principled candidates like Lieberman kiss his filthy hand.) But I don't foresee a quick change to the voting patterns of minorities ahead, and certainly not a shift on par with what abolitionism wrought.

Besides, I bet in the 1850s, people thought an implosion of the Whigs would be great for the Democrats. By 1861, a Republican -- you know, the third party -- was in the White House.