1 - Why is the Necessary and Proper Clause a source of ongoing debate? The clause deals with powers that are ambiguous and misinterpreted.
The "necessary and proper" clause is included in Article I, Section 8, of the United States Constitution. After enumerating a number of the powers of Congress, including borrowing money, coining money, regulating commerce, etc, Section 8 of Article I closes with by saying Congress shall have power "to make all Laws which shall be necessary and proper for carrying into Execution the foregoing Powers, and all other Powers vested by this Constitution in the Government of the United States, or in any Department or Officer thereof." That's a quite broad and sweeping statement. Political leaders have differed in how they would interpret and apply that sort of authority.
2 - Congress checks on the power of the presidency by: overriding a presidential reversal or veto.
A case in point in history would be the War Powers Act of 1973. The War Powers Act gave Congress (the legislative branch) power over whether to continue fighting a war. Congress had pursued such a resolution after President Nixon had ordered bombings in Cambodia during the Vietnam War, without ever consulting Congress or the American people. Nixon vetoed the War Powers resolution, but Congress overrode his veto. Congress asserted that a President cannot commit American forces to military actions abroad without notifying Congress, and limited the time frame a President could keep armed forces deployed without Congressional approval.
