Final answer:
A cloture motion requiring a three-fifths supermajority vote (60 votes) is needed to stop a filibuster in the U.S. Senate, giving the minority substantial power to block legislation.
Step-by-step explanation:
Stopping a filibuster in the United States Senate requires a supermajority vote for cloture. Filibuster is a legislative strategy used by senators to prolong debate and block or delay legislative action on a bill. To cease a filibuster and proceed to vote on a bill, a cloture motion must be passed. Historically, a two-thirds supermajority was required to invoke cloture, but since 1975, this has been lowered to a three-fifths supermajority, equating to 60 votes.
This procedural requirement has empowered the Senate minority with considerable leverage to obstruct bills, if they so choose, despite the majority's wishes. Supermajority rules are not always fixed; for example, for judicial nominations, only a simple majority is required for cloture. The annual budget reconciliation act also bypasses the filibuster, allowing certain types of legislation, like the Affordable Care Act's implementing legislation, to be passed with a simple majority.