Final answer:
The President's veto power is granted by the U.S. Constitution, allowing the rejection of legislation. An effective veto threat requires credibility and the potential to influence legislative change. Today's presidents do not use the line-item veto as it was declared unconstitutional.
Step-by-step explanation:
The constitutional foundations of the president's veto power can be found in the U.S. Constitution, which explicitly grants the President the power to veto legislation passed by Congress. When Congress sends a bill to the president, there are several options at hand. The President can sign the bill into law, veto the bill and return it to Congress with a veto message, let it become law without a signature if not signed within ten days, or enact a pocket veto if Congress adjourns within that period.
An effective veto threat requires a credible likelihood that the veto would be upheld, meaning that Congress is unlikely to override the veto with the required two-thirds majority in both chambers. The effectiveness of a veto threat also lies in its potential to influence legislative action and negotiate changes in the proposed legislation.
At times, Congress might intentionally pass a bill that it knows the president will veto. This can occur for strategic reasons, such as making a political statement or putting pressure on the President. Known as "veto bait," it can put the President in a politically difficult position, especially during an election period.
As for the line-item veto, today's presidents do not use it because it was declared unconstitutional by the Supreme Court following an attempt to grant this power in the late 1990s. Therefore, Presidents must accept or reject bills in their entirety, without the ability to veto specific portions of a bill.