In attempting to make great on his electoral triumph, President Trump will not have the reflexive support of party stalwarts on Capitol Hill that his recent predecessors have enjoyed. His triumph in the Republican primaries was a hostile takeover. He treated congressional Republicans and their leaders with contempt throughout the campaign. Many of them made clear the feeling was mutual, and some refused to support him.

The bruises will heal to some extent—and practicing politicians have to be impressed at how the outsider’s bold proposals and roughhouse style attracted millions of new voters. Yet sharp differences will remain. While some of the president-elect’s positions are solidly Republican (ObamaCare replacement, tax reduction, deregulation), others are nervous-making departures (immigration) and some are outright heresies (trade protectionism, antitrust activism, public-works projects). And Mr. Trump’s aversion to entitlements reform has undercut House Speaker Paul Ryan’s long and careful preparations for finally facing up to the problem.

Under the circumstances, Congress is bound to recover and assert many of its long-neglected legislative prerogatives. In recent decades, our scheme of separated powers has been supplanted by party solidarity between presidents and their congressional co-partisans. (“Separation of Parties, Not Powers” is the title of an influential 2006 study of this development by Daryl J. Levinson and Richard H. Pildes.)

Members of Congress have increasingly acted out of loyalty to party rather than to Congress as an independent constitutional branch. They support or obstruct administration initiatives along partisan lines, and when in support they receive fundraising and bureaucratic favors from the president in return.