Pete Hegseth, President-elect Donald J. Trump’s pick to lead the Pentagon, on Tuesday repeatedly denied misconduct allegations against him and refused to say whether sexual assault, excessive drinking or marital infidelity should disqualify someone from serving as defense secretary.

Under questioning from Senator Tim Kaine, Democrat of Virginia, Mr. Hegseth rejected what he said were “false claims” about his conduct.

He testified to the Senate Armed Services Committee that he was a changed man, while at the same time denying all allegations of wrongdoing against him as a smear campaign orchestrated by a left-leaning media and anonymous sources.

He also refused to say whether a person who had committed the offenses of which he has been accused would be fit to lead the Pentagon.

“You can’t tell me whether someone who has committed a sexual assault is disqualified from serving as secretary of defense?” Mr. Kaine responded.

“Senator, I know in my instance — and I’m talking about my instance only — it was a false claim,” Mr. Hegseth replied.

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