They can override a presidential veto with a 2/3 majority vote in both assemblies of congress.
The President checks Congress when he vetoes a bill
Congress doesn't have the power of veto; the President has the power of veto.
Balance of power.
The bill only goes back to congress if the President vetoes the bill. The President lists the reasons he would not sign the bill.
If the president vetoes a bill, then Congress can override that vetoe, but the bill must go back to Congress to be approved with a majority vote.
He vetoes the bill.
If the President vetoes a bill, it can be passed over his objection by a vote of 2/3 of each house of Congress. This is known as "overriding" the President's veto, and in this case, the bill becomes law even without the President's veto.
Only the President has the power to veto federal bills. He vetoes a bill by sending it back to Congress with his objections.
After the president vetoes a bill, it goes back to Congress. Congress can try to override the veto with a two-thirds majority vote in both the House of Representatives and the Senate. If Congress successfully overrides the veto, the bill becomes law despite the president's objection. If Congress does not override the veto, the bill does not become law.
To overturn the president's veto of a bill 2/3rds of the congress needs to approve.
Overriding the president's vetoes, voting in a bill, & declaring war.
The principle is called checks and balances