I have the vague impression that they had a more gradualist view on slavery and eventually were absorbed into the Republican and Democrat party; however, I must admit my knowledge of American History between 1812 and the Civil War is cursory at best. I am also curious as to how long American had three political parties? Was the dissolution of the Whigs sudden and complete or was it a drawn out affair?
Answer by Eric Ames
I can give you at least a partial answer. Regarding slavery, Whigs generally divided into Cotton Whigs who represented interests tied to the cotton industry, and Conscience Whigs who opposed slavery in almost all cases. Their demise was somewhat drawn out, but the Whigs' division on this issue caused problems in 1852 when they nominated Winfield Scott, whose opposition to slavery damaged him in the South, and didn't help much in the North, where the national party's broadly pro-slavery position wasn't exactly a plus. This was the last presidential election featuring a Whig candidate, and southern Whigs started to leave the party after this point. Opposition to the Democrats in the 1856 election came in the form of John C Fremont of California of the brand new Republican Party. This was more or less the end of the Whig Party, as northern Whigs were absorbed into the Republican Party over the course of the 1850s. The GOP at the time was the political organizing force for the North's anti-South, anti-slavery coalition. The last gasp of the southern Whigs came in 1860, when they formed the new Constitutional Union Party under the former Whig politician from Tennessee, John Bell, who carried Virginia, Kentucky, and Tennessee. There were other important issues in end of the Whigs apart from slavery, but this is just one part of the answer. There's a whole tome on this subject titled The Rise and Fall of the American Whig Party, which I unfortunately have not yet had occasion to read.
Regarding your second question, third parties after the Civil War, as I understand it, were generally weaker than they were before it. The ones that did appear generally didn't last very long. Horace Greeley's Liberal Republican Party allied itself with the Democrats in 1872, but lost to Grant anyway. The People's Party that represented the Populist movement, if memory serves, lasted a bit longer than the Liberal Republicans, but basically got absorbed by the Democrats under William Jennings Bryan. This was followed by a period when the Socialist Part could get as much 6% of the popular vote in a presidential election, but gained relatively little traction. The last nationally important third parties prior to the New Deal era were the short-lived Progressive Parties that formed under Teddy Roosevelt in 1912 and Bob LaFollette in the 1920s. After that, there were independent/third party candidates- Ross Perot, George Wallace, etc.- who would get a healthy chunk of the popular vote and some electoral votes, but did not have much in the way of longevity.