The disintegration of both the Democrats and the Whigs as national parties was both a sign and a cause of the rupture that brought on the Civil War.
I was running some numbers while working on my book, and while the history is a staple of the best commentary on the period, putting it in graphical form really jumped out at me. The popular vote era (i.e., when most states began selecting their electors by a popular vote that was tabulated and published) began in 1828, when all but two states (South Carolina and Delaware) held a popular vote for president. From 1832-60, only South Carolina continued the old method of the legislature selecting electors. For most of this period, the country was evenly divided between slave states and free states, although neither their populations nor their actual electorates were the same. And any measure of polarization between the two is understated by the absence of South Carolina, the most militantly pro-slavery state from the 1832 nullification crisis to its secession in December 1860. Still, we have enough voting data to measure the degree of divergence between the free-state vote and the slave-state vote in presidential contests.
The columns on the far right represent how far Democrats ran ahead in the slave-state popular vote compared with the free-state popular vote, both in raw vote percentage and in the two-party vote (if we exclude third-party candidates, who drew a significant share of the free-state vote in 1832, 1848, and 1856, and the slave-state vote in 1856 and 1860). I have combined here the regional Whig vote in 1836 (when the party ran an eccentric strategy of regional favorite-son candidates in different states) and the two Democrats in 1860 (when the party nominated Stephen Douglas and a Southern rump bolted the convention and nominated John Breckenridge):
At first glance, the immediately striking thing is that the electorate was strongly sectionally polarized in 1828 and 1832, when the Democrats ran Andrew Jackson of Tennessee (originally from North Carolina) against incumbent John Quincy Adams of Massachusetts and then against Henry Clay of Kentucky (originally from Virginia). Jackson’s support was national, with free-state popular majorities in his favor in both elections, but he was much stronger in the South against Adams, and continued that advantage even when running against a fellow border-state Southerner. Given that neither election was contested around slavery — the big issues were about banking and tariffs and executive power — that reflects a combination of Jackson’s regional populist appeal with the poor organization of the Southern Whigs (the Whig Party was only formally organized in 1834, but had grown out of the Adams and Clay campaigns fused with the last vestiges of the Federalist Party).
When New Yorker Martin Van Buren succeeded Jackson in 1836 and ran against the regional Whigs, the Jackson edge in the South evaporated, and we entered an era of five elections in which free/slave state polarization was minimized. True, Democrats won three popular majorities in the slave states in this period, and only one in the free states. But the Whigs, in both of their victories (in 1840 and 1848) also won majorities among slave-state voters. The parties were aware enough of the issue that it shaped their choice of candidates. Democrats ran free-state nominees in six of these seven elections after Jackson: Van Buren twice, Lewis Cass of Michigan in 1848, Franklin Pierce of New Hampshire in 1852, James Buchanan of Pennsylvania in 1856, and Douglas of Illinois in 1860 (Breckenridge was from Kentucky). In fact, after James K. Polk in 1844, unless you count Breckenridge or Woodrow Wilson (who was raised in Virginia but ran after being governor of New Jersey), the Democrats didn’t nominate a candidate from the former slave states until Harry Truman in 1948. (It reflects the shifts in regional polarization that they’ve done so seven times since then, in running Lyndon Johnson, Jimmy Carter, Bill Clinton, Al Gore, and Joe Biden). After Van Buren, no Democrat would win a popular majority in the North until Franklin D. Roosevelt in 1932, which helps explain why FDR was also the first Democrat to win a national popular majority since Pierce in 1852.
The Whigs tried something like the opposite strategy, running William Henry Harrison (a slave-owning Virginian, albeit since an officeholder in Indiana and then Ohio) in 1840, Clay in 1844, Zachary Taylor (a Virginian who owned a slave plantation in Louisiana) in 1848, and Winfield Scott of Virginia in 1852. Notably, however, all the Whig nominees were more identified with the federal government than with one section, with three of the four being career Army officers.
When the Republicans replaced the Whigs, they didn’t bother. The southern arm of the Whigs had collapsed so completely, and the Republican Party was organized so thoroughly around opposing the expansion of slavery, that while Republicans sought moderate anti-slavery men who had left the South (John C. Frémont was born in Georgia and raised in South Carolina before heading west; Abraham Lincoln was born in Kentucky but grew up in Indiana before moving to Illinois), they were undeniably anti-slavery men. The result was a massive acceleration of polarization, as Republicans won a plurality of the free-state vote while losing the 1856 election, and won a majority of the free-state vote and carrying every free state in 1860, while winning nearly no votes in the slave states. This also shifted the dynamics of third-party votes. Before 1856, third-party candidates prospered in the free states by taking a harder anti-slavery line than the national Whigs could afford; by contrast, Millard Fillmore’s Know-Nothing campaign in 1856 and John Bell’s Constitutional Union ticket in 1860 sopped up the votes of former Whigs in the South who were made politically homeless because they couldn’t bear to support an anti-slavery northern ticket.
The net result is that the disintegration of both the Democrats and the Whigs as national parties, combined with the rise of the Republicans as a purely sectional party that could nonetheless manage an Electoral College majority, was both a sign and a cause of the rupture that brought the war.