Slavery would have been abolished in America through a civilized political process ... if the South hadn’t seceded.
From America’s founding, slavery was the great dividing issue. It turned what should have been simple matters into violent controversies. The accepted wisdom of the pro-slavery Democrats and the moderate Whigs was to maintain a careful balance of power: free state for slave state, divide the Senate equally and create deadlock.
Under this arrangement the South got to go on using human beings as chattel and the North got to, well, not be completely overrun poltically. But the abolitionist movement grew in fervor and in power, and eventually tore apart the Whig Party.
The Republican Party rose from the ashes, driven in large part by abolitionists yearning to destroy slavery in America. Because the North was more populous than the South, they had control of the House, and the abolitionists had hope of passing anti-slavery laws there.
Due to the agonizing balancing of the states, the abolitionists had no hope in the Senate. But if they could get their own into the Executive Branch, they wouldn’t need it. Anti-slavery laws, passed in the House, could get a tied vote in the Senate, which the anti-slavery vice president could break, and so send the law to the anti-slavery president to sign ...
Every president elected by the Democratic Party would, of course, defend slavery. No Whig president would attack it, which in practice amounted to the same thing. The presidency reverted between the pro-slavery faction and the moderate faction, and slavery remained safe.
Then the Whigs fell. The Republican Party - newborn and driven with anti-slavery conviction - won the presidential election of 1860, and slavery was no longer safe.
And then the South seceded.
Had the South not seceded, the Democrats would have had a 39 to 29 majority in the Senate of the 37th Congress. House was 108 D and 107 R. Although some of those Democrats may have been anti-slavery.
The balance of slave to free states had been lost in 1850 with the admission of California. By 1860 there were three more free than slave states, and KS was obviously about to join up, with no future slave states in realistic prospect.