Greg Weiner, an associate professor of political science at Assumption College, is the author of “Madison’s Metronome: The Constitution, Majority Rule, and the Tempo of American Politics.”

As the core of the Republican Party that is independent of President Trump is corroded by retirements — Sens. Jeff Flake (Ariz.) and Bob Corker (Tenn.) most recent among them — the problem is not the decay of the moderate or independent element in Congress. It is the decay of the congressional element in Congress. The problem pertains far less to opposition to this president than to the long-range erosion of congressional resistance to the presidency as an institution. With defenders of congressional prerogative departing and their remaining colleagues orienting themselves in subservience or opposition to the White House, we are witness to the cementing of the presidency as the motive force in the American regime.

The retiring senators have been voices for congressional power. Corker, for example, asserted Congress's right to review the Iran nuclear deal. Flake's retirement address to the Senate channeled James Madison's case for the separation of powers in calling on Congress to act on its institutional rather than partisan interests, joining across party lines to defend its own powers rather than dividing according to policy preferences. Flake has thus worked with his Democratic colleague Tim Kaine of Virginia on an effort to reclaim Congress's war power by calling for a repeal of the post-9/11 Authorization for Use of Military Force and replacing it with a more restrained resolution dealing with the Islamic State.

Sen. Jeff Flake (R-Ariz.) warns populism is not a governing philosophy. (Video: Dalton Bennett, Kate Woodsome/The Washington Post)

Contrast that institutionalist behavior with those who remain. Senate Majority Leader Mitch McConnell (R-Ky.) rushed to the Senate floor a debt-ceiling deal Trump struck with the Democratic congressional leadership. The point is not the deal's merit but rather McConnell's reasons for pushing it: He was doing so, he said, "based on the president's decision." Senate Majority Whip John Cornyn (R-Tex.), asked recently about his position on a legislative issue, replied that he was "with the president." Asked where the president was, Cornyn reportedly threw his hands in the air. Democrats similarly acceded to President Barack Obama on questions of executive power, a tactic whose danger they are now learning as they object to Trump's assertions of unilateral authority today.

This wholesale deference to the president is entirely contrary to constitutional expectation. Madison could not have even conceived of it. He based the system of separation of powers on the assumption that members of each branch of government would defend their institutional prerogatives because they would want to exercise power. What we have instead is outcome-based constitutionalism: a disintegration of the norms according to which it once mattered not just what happened, but how it happened. And whether a given policy is enacted by presidential fiat or by the steady and deliberate workings of Congress matters a great deal.

The idea of the separation of powers — the absence of which Madison called "the very definition of tyranny" — is to prevent any one individual or institution from exposing the citizenry to arbitrary power by controlling all the levers of government. Yet that is what today's presidency commands: Presidents, by their signatures, direct vast swaths of domestic policy and questions of war and peace alike.

Citizens should object to this regardless of their views of a particular president. In the diversity allowed by a membership of 535, Congress more fully represents the richness of U.S. politics than the presidency, a binary institution with which, at a given moment, one either agrees or does not. This diversity also disperses rather than centralizes power. And Congress is usually institutionally incapable of the impetuosity that tempts presidents who can convert whims into policy.

It is true that Congress is increasingly unable to concur on legislation. In many ways, and for myriad reasons, that is a problem. But it is worth recalling that Congress's job is not to legislate. It is to represent what Madison called "the cool and deliberate sense of the community." Congress is designed to move slowly in order to dissipate passions and allow the public's reason to take hold. It is not supposed to be able to generate major legislation on which broad and sustained public consensus does not exist.

To be sure, Congress has been in decline for generations. What is alarming recently is the extent to which the organizing principle of Congress is members’ attitude toward whatever administration is in office. This phenomenon may be inflamed by the intense feelings Trump arouses, but it is hardly confined to him.

In a reversal not only of the Founders’ expectations but also of their hopes, the presidency is now the sun around which all constitutional bodies orbit. At intervals, that is convenient for either side’s policy preferences. But if constitutionalism commends any principle, it is for all players in the system to remember that today’s winners will be tomorrow’s losers. Institutions are our hedge. Bargaining them away for temporary advantage — a game both sides have played — is a loser’s gamble.

Read more on this issue:

Catherine Rampell: How Republicans can get everyone to stop blaming them

George F. Will: Trump may awaken a slumbering Congress

Kenneth W. Starr: Firing Mueller would be an insult to the Founding Fathers

George F. Will: Trump’s violations of federalism would make Obama jealous