by Steve Rankin
The Republican Party was founded in the summer of 1854, and it had the largest number of members in the US House that met in 1855, so it was a third party for only a few months. (Of course, with the draconian ballot access laws, that would be impossible for a new party to pull off today.)
Abe was elected in 1860 with 39.9% of the popular vote, thanks largely to the North-South split in the Democratic Party.
Illinois Sen. Stephen Douglas, the nominee of the northern Democrats, actually finished fourth in electoral votes in 1860. Vice President John Breckinridge of Kentucky, the southern Democratic presidential nominee, was second in electoral votes, and Tennessee's John Bell of the Constitutional Union Party was third.
Steve Rankin's blog is Free Citizen (on The Hankster sidebar).