Started in the Civil War. Power was concentrated in the New York/Washington DC corridor, and has remained there ever since.
No, started long before 1861.
Before 1791 New York City was the US capital, President Washington was inaugurated there.
Washington's mostly Northern Federalists were the majority party.
After 1800 Washington, DC, became the US capital and Jefferson's Southern Democrats ruled the majority, until 1861.
Democrats by their nature love ruling and go nuts when thrown out of power.