The US with a multiparty system

[Thunderbird is the word]

What would an American electoral map look like in a world with say a five party system. Democrats and Republicans would still exist but with Libertarians and Greens regularly getting 10-15% of the vote and serving as junior partners? Also you'd have a fiscally liberal+socially conservative populist/nationalist party.

[Unconditional Surrender Truman]

For starters, let's assume that, in the 2012 Election, Johnson and Stein win a combined 31.4% of the popular vote (compared to 1.4% in the actual election), and that the pv totals of the two major parties is adjusted proportionally to reflect that change. Let us in turn assume that the number of votes for third parties in each state increases in proportion to the national increase (ie: Johnson's national vote total increased by 16.2% compared to the actual election, so his total would increase by 16.2% in every state). The result would be something like this:

Obama / Biden (DEM) 298 EV; 35.0% pop. vote
Romney / Ryan (REP) 229 EV; 33.2% pop. vote
Johnson / Gray (LIB) 11 EV; 15.99% pop. vote
Stein / Honkala (GREEN) 0 EV; 15.36% pop. vote

The increased third party vote allows Johnson to carry Alaska, New Mexico, and Montana and flips Florida to Romney; otherwise, the electoral map is unchanged (Stein fails to win any states). In essence, we are left with a map that is not too different from reality, with the exception that most states are carried by pluralities of 30-40%. Only in states where a third party candidate won 2% of the vote or more, and no major party candidate won more than 55%, is a third party able to carry a state, by varying margins.

[Kingpoleon]

189: Sen. Marco Rubio(R-FL)/Gov. Chris Christie(R-NJ)
189: Sen. Claire McCaskill
80: Gov. Dino Rossi(L-OR)/Rep. Justin Amash(L-MI)

189: Sen. Charlie Crist(R-FL)/Gov. Abel Monando(R-CA)
189: Sen. Joseph Manchin(D-WV)/Sen. Claire McCaskill(D-MO)
80: Gov. Howard Dean(G-VT)/Sen. Cory Booker(G-NJ)
80: Rep. Justin Amash(L-MI)/Gov. Dino Rossi(L-WA)

[bagelman]

I think it would involve a lot of coalitions and proportional states like Maine and Nebraska. The office of vice president would be much more important than it is now, and gerrymandering would be more of a serious issue.