The administration is showing it doesn’t view the House and Senate as equal partners. So far, Republicans, who hold both majorities, are accepting their new status.

For the last 50 years, almost every nominee to lead the Pentagon has been a consensus pick who drew lopsided margins of support in the Senate. Pete Hegseth’s nomination has been a notable exception.

The large domed expanse, a breathtaking and solemn space in the Capitol, was the scene of mayhem wrought by Trump supporters four years ago. A very different crowd of his backers will be on hand Monday.

“Your biggest issue is the perception of age,” Mike Donilon, the president’s longtime strategist, told him in 2022, according to people who heard him.

Opposition to prospective nominees, along with time-consuming Senate rules, will slow floor action despite the Republican push for approval of some of President-elect Donald J. Trump’s picks as soon as Jan. 20.

Democrats are demanding that Republicans slow consideration of picks for the new administration until they can review background checks, as the G.O.P. faces major pressure to quickly confirm them.

Bipartisan legislation enacted in 2022 makes it much more difficult for lawmakers to object to a state’s votes, and clarifies that the vice president cannot unilaterally overrule them.

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.

The question of whether the president-elect would back the speaker for another term has hung over Republicans, especially after recent tensions. But the endorsement does not lock up the job for Mr. Johnson.