Susan Collins declares her voting intentions on Friday

US President Donald Trump's nominee to the Supreme Court appeared all but certain to win approval from the Senate after a key lawmaker backed him.

Susan Collins, a member of Trump's Republican Party who was among the last to declare her voting intentions, said Friday allegations that Brett Kavanaugh had sexually assaulted a fellow teenager nearly four decades ago were unproven.

"I do not believe these charges can fairly prevent Judge Kavanaugh from serving on the court," Collins told lawmakers in a speech from the floor of the chamber.

"The presumption of innocence and fairness do bear on my thinking and I cannot abandon them," she added, calling Kavanaugh "an exemplary public servant."

(With inputs from agencies.)