To be confirmed as health secretary, Robert F. Kennedy Jr. can afford to lose no more than three Republican votes if all Democrats are united in their opposition to him.

Senators Lisa Murkowski, Susan Collins and Mitch McConnell voted against confirming Donald Trump’s pick for defense secretary. To Capitol insiders, their decisions weren’t surprising.

There are an estimated 300,000 polio survivors in the United States. For some, the nomination of Robert F. Kennedy Jr. as health secretary is reviving their painful memories.

The president-elect and his allies have spent four years reinventing the Capitol attack — spreading conspiracy theories and weaving a tale of martyrdom to their ultimate political gain.

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.