With Mr. Trump putting forward a raft of contentious prospective nominees and threatening to challenge congressional authority in other ways, Republicans who will hold the majority come January could find themselves in the precarious position of having to choose between standing up for their institution or bowing to a president dismissive of government norms.

The clearest and most immediate point of tension is likely to be Mr. Trump’s efforts to skip the Senate’s traditional confirmation process to install loyalists, including some with checkered backgrounds, in his cabinet. But the president-elect has also signaled he expects Republicans on Capitol Hill to accede to his wishes on policy, even if that means ceding Congress’s control over federal spending. Both are powers explicitly given to the legislative branch in the Constitution.