After picking up seats in West Virginia, Ohio and Montana and winning an unexpectedly close race in Nebraska, the G.O.P. sealed a majority. Tight races in swing states will determine their margin.

Republican control could be a moderating force on Mr Trump. The Senate has welcomed more right-populist Republican members like Mr Moreno in recent years, but still remains a bastion of pre-Trump conservatism. A narrow Republican majority in the Senate could empower moderates to reject Trump nominees outside the political mainstream.