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It is not part of the US Constitution, becoming theoretically possible with a change of Senate rules only in 1806, and never being used until 1837.
[60] Rarely used for much of the Senate's first two centuries, it was strengthened in the 1970s
[61] and in recent years, the majority has preferred to avoid filibusters by moving to other business when a filibuster is threatened and attempts to achieve cloture have failed.
[62] As a result, in recent decades this has come to mean that all major legislation (apart from budgets) effectively now requires a 60% majority to pass
On November 21, 2013, the then Democratic controlled Senate exercised the
nuclear option, in a 52 to 48 vote, to require only a majority vote to end a filibuster of all executive and judicial nominees, excluding Supreme Court nominees, rather than the 3/5 of votes previously required.
[63] On April 6, 2017, the Republican controlled Senate did the same, in a 52 to 48 vote, to require only a majority vote to end a filibuster of
Supreme Court nominees.
[64] A 60% supermajority is still required to end filibusters on legislation.