That's because they are.
They lived in the same geographical locations as modern Democrats, their ideology and that of Modern Democrats matches quite closely, and their demographic backgrounds are very similar. (elite wealthy, urban, progressive)
People refuse to believe the parties switched ideologies over the years, but there is plenty of evidence to show what they actually did was just switch names.
The 1860 Republicans were Northern, Liberal, elites living in Urban environments, they supported High Taxes, massive spending, protectionism, and "social change."
They were social activists. The Progressive Era features an endless train of Republican Women (women couldn't vote, but their families were all Republican) activists trying to re-engineer society. Among them were Margaret Sanger, Elizabeth Cady Stanton, and Susan B. Anthony.
John Brown was a traitor and a terrorist.