Northern Democrats were older than the upstart Republicans, at least in political years. They tended to be Catholics, Lutherans, or other no-evangelical Protestants. They favored low tariffs and minimal government. They were reluctant to take a stand against slavery, even at the height of the Civil War, because they supported the principle of States Rights. In places like New York State, they were also masterminds of the political machine.
Their symbol, the donkey, originated in 1828 during the campaign of Andrew Jackson, whose opponents called him a jackass. He exploited the label to emphasize their stubbornness.
In the South, the Democrats were the old Confederates. They were willing to humbly apologize for their roles in the Civil War and begged for forgiveness from President Andrew Johnson, especially when they meant they might get their old lands back again. But they never gave up their basic beliefs in states' rights, their devotion to the Old South, or their opposition to abolishing slavery. In their minds, blacks would always be inferior, 3/5th human beings, and their political goals centered on wrestling political control away from blacks to restore it to the hands of the old slave-based aristocracy.
In the 1870s, the Ku Klux Klan and similar organizations became the terrorist arm of the Democratic party in the South. But they will need a post of their own.