WASHINGTON — Senator Susan Collins was on her own.

Her usually reliable Republican ally, Senator Lisa Murkowski of Alaska, had just broken with the party against the confirmation of Judge Brett M. Kavanaugh to the Supreme Court. That left Ms. Collins as the sole Republican supporter of abortion rights who could derail a man seen as a serious threat to Roe v. Wade — not to mention that he had been accused of sexual misconduct.

Ms. Collins did not derail him.

Instead, she took to the Senate floor Friday afternoon and delivered a reasoned, carefully researched, 45-minute point-by-point defense of her support for Judge Kavanaugh.

“We’ve heard a lot of charges and countercharges about Judge Kavanaugh,” Ms. Collins said. “But as those who have known him best have attested, he has been an exemplary public servant, judge, teacher, coach, husband and father.”

[Read Ms. Collins’s full speech declaring support for Judge Kavanaugh.]

As for the accusations against him, she said, “In evaluating any given claim of misconduct, we will be ill-served in the long run if we abandon the presumption of innocence and fairness, tempting though it may be.”