The Senate and the House of Representatives can override the President's veto by blank vote
A majority
B 3/4
C 3/5
D 2/3

The process by which each chamber of Congress votes on a bill vetoed by the President. To pass a bill over the president's objections requires a two-thirds vote in each Chamber. Historically, Congress has overridden fewer than ten percent of all presidential vetoes.

Yes, reasons why is sectionalism compromise of 1850 and secession i hope this