First, Congress should seize the initiative on issues where it knows Mr. Trump is wrong. We’ve seen encouraging signs of this already: Congress refused to repeal Obamacare, albeit by the slimmest of margins; imposed sweeping sanctions on Russia; and has explored how to protect the independence of the special counsel. It could do far more. House Speaker Paul D. Ryan (R-Wis.) and Senate Majority Leader Mitch McConnell (R-Ky.) must know it makes no sense to torpedo successful trade pacts with reliable U.S. allies such as Canada, Mexico and South Korea. They could, at a minimum, hold hearings to make that case. Surely congressional leaders understand the United States would weaken itself if it sharply reduced Bush-era programs to counter AIDS, polio and other diseases in Africa. Right now Congress could act to save the nearly 700,000 young “dreamers” from the deportation that would undeservedly ruin their lives. It could hold hearings on the rise of white-supremacist organizations.

Second, congressional leaders can offer a contrast to what Mr. Corker described as the “adult day care center” at the White House simply by presiding over their branch with institutional dignity and respect for tradition.