Yeah no that’s what happened. The Democratic Party was run completely by Southern slave holders, while only half the Whigs were based in the slave power. By the end of the Mexican American War, John Calhoun was Democratic in all but name, and this wonderful phrase was in the Democratic Platform of 1848
“All efforts of the Abolitionists or others made to induce Congress to interfere with questions of slavery, or to take incipient steps in relation thereto, are calculated to lead to the most alarming and dangerous consequences; and that all such efforts have an inevitable tendency to diminish the happiness of the people, and endanger the stability and permanence of the Union”
Not that there were any Abolitionists in elected office. This was when William Lloyd Garrison was being dragged through the streets of Boston by a horse for publishing the Abolitionist paper The Liberator. The Whigs saw a faction evolve within them called “The Conscious Whigs”, who thought that slavery was bad. These Whigs were so marginalized by the establishment Whigs like Henry Clay that they formed a coalition with the Northern Democrats, who fought to support the poor but also were completely absent from Federal Congress. That coalition would go through a few labels but ultimately became the Republican Party.
As the 1850s rolled through, things only got worse. Democrats undid every compromise Henry Clay had brokered so that they could expand slavery into as many territories as they could. The Kansas Nebraska Act was a piece of Democratic legislation that declared Kansas’s Free/Slave status would be decided by popular vote, leading to terrorism against any abolitionist residents by the slave holding settlers. As a result, radical conscious Whig Senator Charles Sumner gave the speech “The Crimes Against Kansas,” a piece of rhetoric so firery that Democratic Representative Preston Brooks beat him with his cane the next day, and he couldn’t return to work for three years due to physical and psychological injury. When John Brown began to lead his militia to strike back, the Democratic Pierce administration sent in soldiers to put it down, continuing to ignore the slaver terrorism (Brown would have perished here but the officer Pierce sent was cousin to one Mr Charles Sumner and still bitter about the cane thing).
Come 1861, it was Democrats who led the calls to secession, but we’ve talked about that on here enough I don’t feel I need to repeat myself about the cause of that war. In the wake of the war, the Republican Federal Government went through great strides to reach for political equality for the freed slaves, but were repeatedly stopped by white terrorism by Democrats. The Acts of Reconstruction, which put the South under martial law until they got their act together, were repealed due to Democrats calling for a second Civil War in 1876 (President Grant an Sec. War Sherman must’ve found that one strange, but it worked) over a Presidential election where the Republicans had won by one vote.
What they always leave out is that the Democrats were punished for their incendiary tactics, unable to obtain the White House for 50 years (save for Cleveland, who the Party didn’t like anyway) as a result of their secession. By the 1890s, the second Populist movement lead by William Jennings Bryan demonstrated that the Democrats were a class conscious party looking to start a welfare state and care for those in need.
I’m gonna close with this quote from Mr John R Lynch, former slave and Federal Representative:
“The fundamental principle of the Republican party, – the one that gave the party its strongest claim upon the confidence and support of the public, – is its advocacy of equal civil and political rights. If that party should ever come to the conclusion that this principle should be abandoned, that moment it will merit, and I am sure it will receive, the condemnation of the repudiation of the public”
EDIT: Today is also the 161st anniversary of Brooks caning Sumner, oddly relevant to this discussion.