John Thune Takes Charge in the Senate, Ushering in a New Leadership Era

John Thune won election to the Senate in 2004 by conquering Senator Tom Daschle, the powerful Democratic majority leader, only to quickly face a mortal threat to his political future when a major Air Force base in his state was threatened with closure.

He beat back the effort and saved Ellsworth Air Force Base, a financial engine in his largely rural state, keeping his job and learning valuable lessons that have helped power his rise in the Senate.

Now the South Dakota Republican has ascended to the top and will become majority leader in the new Congress that convenes on Friday, but he is again facing an early challenge and a major threat to his political standing.

Mr. Thune’s task this time is to juggle President-elect Donald J. Trump’s demands, the competing desires of 53 G.O.P. senators and a formidable legislative agenda. It starts with shepherding multiple baggage-laden Trump nominees to confirmation in the closely divided Senate, where he can afford to lose no more than three Republican votes if Democrats hold together in opposition.

At the same time, he intends to use a tricky maneuver to steer around a filibuster and pass a combination border security, military spending and energy production bill that will require serious legislative finesse. And he wants to do it in the first month or so, while also reordering how and how often the Senate works.