Senator Susan Collins, who supports abortion rights, endorsed Brett Kavanaugh’s nomination for the Supreme Court. It has emerged as a major issue in her campaign for re-election.

Mr. Platner’s withdrawal from a marquee Senate race, under pressure from his party, has set in motion an unusual competition to become the new nominee.

The vote to open a war powers debate, a pair of attempted veto overrides and a split on health care suggested a greater appetite among Republicans to challenge the president.