Senators from both parties agree that one cause of the trends is the ease with which lawmakers can now bring the institution to a halt. Once the Senate found a way to move on to other business while a bill was being filibustered, senators faced little personal pressure against mounting one. And when the number of votes needed to break a filibuster dropped to 60 in 1975, from 67, the Senate minority could claim that this change allowed for reasonable bipartisan compromise.
But there have been major cultural shifts as well, past majority leaders and academics say. Lyndon B. Johnson once said the Senate was an ecosystem of whales and minnows. Get the few whales and the minnows follow.
The advent of C-Span 2, which put cameras in the Senate in the 1970s, helped turn all the minnows into whales in their own right, Ms. Bell said. The influx of House Republicans in the 1990s, steeped in the partisan fights of Mr. Gingrich, furthered the shift. Republicans and Democrats alike point to a moment in the 1990s when Rick Santorum, then a Republican senator from Pennsylvania and a former House warrior, refused to yield the floor to a colleague when asked, a refusal almost unheard of in the Senate.