WASHINGTON — The nomination of Judge Brett M. Kavanaugh to the Supreme Court has exposed just how far the Senate has drifted from the rules of decorum that once elevated senatorial prerogative over party, leaving behind the kind of smash-mouth partisan politics that have long dominated the unruly House.

Senate rules dating back to Thomas Jefferson mandate that lawmakers refer to each other by state and title — “my good friend, the senator from California” — and forbid members from questioning motives, maligning a home state or imputing “to another senator or to other senators any conduct or motive unworthy or unbecoming a senator.” Senators are not even supposed to read a newspaper while another member of the body is speaking on the chamber floor.

Few of such niceties have been in evidence as the Senate struggles to fill the Supreme Court seat of the retired Justice Anthony M. Kennedy. Republicans have accused Democrats on the Judiciary Committee of plotting a last-minute smear of Judge Kavanaugh, and have privately argued that the party’s senators demeaned themselves and the body by asking a nominee to the Supreme Court intimate questions about his drinking habits and sexual behavior.

With only circumstantial evidence, Senator Tom Cotton, Republican of Arkansas, called for an investigation into whether Senator Dianne Feinstein of California, the committee’s ranking Democrat, sat on and then leaked Christine Blasey Ford’s letter accusing Judge Kavanaugh of sexual assault. Senator Lindsey Graham, Republican of South Carolina, has delivered an escalating series of threats to his Democratic counterparts.