The affable traditional conservative from South Dakota is the first new chief of the Senate G.O.P. in nearly two decades. He will confront a challenge managing President-elect Donald J. Trump’s expectations.

New Senate Majority Leader John Thune is kicking off the new session with a promise to preserve the filibuster, saying in his first floor speech as leader that his priority will be to “ensure the Senate stays the Senate.

The Senate gaveled in on Friday and officially came under Republican control, with newly elected Majority Leader John Thune (R-SD) being sworn in and vowing to preserve the filibuster, among other