In 1840 a new party, the Liberty Party, started. They were anti-slavery
By 1848 they had become the Free Soil Party
starting after the mexican-american war, the Whigs started to lose support in slave states.
in 1854 everything went to hell due to the kansas-nebraska act.
Whigs lost all reps in slave states, with many pro-slave whigs joining the American or Know Nothing party, which was anti-immigrant.
Free-Soilers joined with two new parties, the Republicans and Anti-Nebraska Parties, under the Anti-Nebraska umbrella.
Anti-Nebraska and Whig parties joined to form the "Opposition"
This would later morph into the Republican Party.
By 1858, southern Whigs, unhappy with the Republicans, as well as southern Know Nothings, formed the Constituional Union Party.
This party would later merge with the Republicans.
Starting in 1860, was the Civil War
1854 really saw things go to hell too.
In 1852 the Senate had 34 Democrats, 20 Whigs, 2 Free Soilers, and 1 Know Nothing.
When congress was sworn in after the 1854 elections, there were 36 Democrats, 9 Whigs, 9 Republicans, 2 Free Soilers, and 2 Know Nothings.
By the swearing in of the congress after the 1856 elections, the Democrats had 37 Senators, the Republicans 20, and the Know Nothings 4.