SEN. BOB CORKER, a Tennessee Republican, says "the vast majority" of Senate Republicans understand the "volatility" of President Trump "and the tremendous amount of work that it takes by people around him to keep him in the middle of the road."

If that is so — and Mr. Corker seems liberated into candor by his decision not to run for reelection next year — how should the Republican caucus make use of that knowledge?

We were critical of Republican leaders from early in Mr. Trump's candidacy for their refusal to stand against the malign sentiments he voiced. It continues to be important, in our view, that they defend tolerance, constitutional norms and other values that Mr. Trump has challenged. But speaking isn't enough; and getting into Twitter battles with the president, as Mr. Corker did Sunday morning, may not be all that productive.

[What if Corker is right?]

One avenue open to Congress would be to remove the president from office. If indeed Mr. Trump is so reckless that he could set the nation "on the path to World War III," as Mr. Corker said Sunday in an interview with the New York Times, this possibility can't be dismissed. "He concerns me," said Mr. Corker, who serves as chairman of the Senate Foreign Relations Committee. "He would have to concern anyone who cares about our nation."

But Congress is not ready to consider such an option — nor, in our view, should it be. Impeachment is an extreme measure that would roil the nation and should be embarked upon only with clear justification. So we repeat: What is the right response for a congressional majority that understands its president is unfit?

It seems to us the answer falls into two baskets. 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.

[The issue that is — unfortunately — uniting Americans on the left and the right]

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. This would include letting Democrats have a say in the debate, in implicit contrast to the president's contempt for those who disagree with him. It would include legislating based on facts and evidence, including the best available guidance from the Congressional Budget Office. Ideally, it would demonstrate that governance based on ideals tempered by compromise, rather than showmanship and cynicism, can produce results.