Both parties bear some responsibility for the changes, experts say, though not in the same precise ways.
Before 1917, senators could delay final votes on legislation by holding the floor and talking. There was no mechanism to stop them, but such filibusters were rare until the debates surrounding entry into World War I. In 1917, the Senate adopted its first “cloture” rule: two-thirds of the Senate could cut off debate on a bill and force a final vote.
Between 1917 and 1971, no session of Congress had more than 10 such votes in its two years. Still, filibusters were common enough that in 1971, Mr. Byrd, a master of Senate procedure, shifted the rules to allow the Senate to take up other legislation during a filibuster.
That change began an escalation of tactics, in which both delays and attempts at circumventing delays have become more common, with one often leading to more of another. Moves by the minority to obstruct bills elicited responses from the majority worsening the environment.
In the 93rd Senate, which met in 1973 and 1974, the number of cloture motions filed — a rough measure of filibuster threats — jumped to 31, from an average of fewer than two per Congressional term between 1917 and 1970. Throughout much of the next two decades, cloture votes continued to rise, regardless of which party was in the minority, with many such motions filed in anticipation of filibusters.