Why the Whig Party Collapsed
...
The divisive slavery issue came to a head again in 1854 with the passing of the Kansas-Nebraska Act, which authorized new territories and states to decide for themselves if they wanted to allow slavery.
Anti-slavery Whigs, deciding that their party wasn’t sufficiently committed to halting the spread of slavery, splintered off and formed the Republican party along with anti-slavery Democrats. Among the former prominent Whigs who turned Republican were Thaddeus Stevens, William Seward and Abraham Lincoln.
Meanwhile, other Whigs were getting swept up in anti-immigrant, nativist movements like the Know Nothings, a secret society that grew to become a political force in the 1850s. Fillmore, who had been dumped by the Whigs in 1852, ran in 1856 as the nominee of the American Party, the political wing of the Know Nothings. Many conservative Whigs followed him.
1856 was the last election in which the Whigs fielded a candidate, but former Whig William Seward, who went on to serve as Lincoln’s secretary of state, pronounced the party’s eulogy in 1855: “Let, then, the Whig party pass. It committed a grievous fault, and grievously hath it answered it. Let it march out of the field, therefore, with all the honors.”
“It’s remarkable how fast it all fell apart for the Whigs,” says Wallach. “From right before the 1852 election thinking they were in good shape, to 1854 being clearly obsolete and in 1855 literally going out of business.
“It’s pretty striking.”
https://www.history.com/news/whig-party-collapse
If we are not coming up to a 1912 Bull Moose party situation, perhaps we are coming up to a 1854 Whig party situation.