The question will test your knowledge of American history.
If you get this question wrong, then perhaps it would be wise to hit the library and educate yourself..
You really should take your own advice. Both Parties had 'pro-slavery' factions, and the northern Democrats split on the issue. In fact, without northern Democratic support, Lincoln's military dictatorship and suspension of the Constitution couldn't have been launched and his illegal war would never have got off the ground, even with his private army of 75,000 troops, paid for with Federal tax money.
Claiming to be 'anti-slavery' is also problematic, since most of the 'anti-slavery' types were more of the 'ship them all back to Africa' types, including Lincoln, and what they were supporting was a policy of no blacks allowed at all in the new territories, i.e. only white settlers allowed; stronger Black Codes were passed in most mid-western states between 1853 and 1857, including Illinois, also supported by Lincoln before he became President, which made it impossible for a black person to make a legal living there.
See also Lincoln's military governors of the Union occupied states during and after the war, where Lincoln and his successor backed his governors' martial laws ordering 'freed' slaves to remain on their plantations and work for 'pay' also dictated by the military governors, a whopping $3 a month, with the added bonus of facing arrest and forced labor as a criminal if caught off their plantations without the express written permission of their new 'employer'. 'Anti-slavery' was a distinction without a difference for most northerners, especially Lincoln. The term 'anti-slavery' is highly misleading, as anybody who has actually hit the libraries and educated themselves.
They can also find a couple of books out there on the 1864 elections and the Republican candidates own speeches and platforms if they have some delusion anything changed during or after the war. Some 300,000-400,000 'freed' slaves died in 'property camps', to keep them from fleeing north as well, so let's not pretend there was much in the way of any real 'anti-slavery' sentiment, just even worse racism, of the 'white nationalist' type of ethnic cleansing.
Trying to pretend the Republicans have some sort of claim to moral superiority over Democrats is just ridiculous nonsense, just a s ridiculous as claiming northerners were on some kind of moral crusade to 'end slavery' and 'end racism n stuff'. Utter drivel.