Mr. Obama does bear responsibility for an “aggressive unilateralism”—as Mr. Boehner puts it—that has stripped legislators of their constitutional role. But he has been indulged in his every excess by legislators of his own party. Call it wimpy, call it the Stockholm syndrome, call it what it is: Congressional Democrats watch supinely as the president treads on their powers. Separate branch of government? Who, us?

Call it too another disturbing reality of the Obama era. In the history of this country, there was one thing on which Republicans and Democrats, House and Senate, could regularly agree: Nobody messes with Congress’s powers. Political parties were happy to rally votes for a president’s agenda, to slam his opponents, to excuse his failings. But should that president step on Congress’s size 12 toes, all partisan bets were off.

Andrew Johnson was impeached by nearly two-thirds of the House for the “high crimes and misdemeanors” of violating a controversial law that the House had passed. Theodore Roosevelt’s regulatory reaches were bitterly opposed by conservatives in his party. The Republican speaker, Joseph Gurney Cannon, famously complained of the Rough Rider: “That fellow at the other end of the avenue wants everything from the birth of Christ to the death of the devil.” When FDR announced his court-packing plan, it was a Democrat, Henry Ashurst, who labeled it a “prelude to tyranny” and delayed the bill in the Senate for 165 days, contributing to its defeat.

This institutional cantankerousness was alive and well through the Bush era. In May 2006, the FBI raided the office of then-Democratic Rep. William Jefferson. Republican Speaker Denny Hastert and Minority Leader Nancy Pelosi issued a blistering joint statement denouncing it as a violation of the separation of powers. Republicans and Democrats spent much of the Bush years jointly attempting to force the president to give Congress more say in his wars and detention policies.

 

Sen. Harry Reid and Rep. Nancy Pelosi on Capitol Hill in April. Associated Press

Scholars can and do argue over the constitutional merits of these episodes. What’s clear is a long history of Congress vigorously slapping back at any intrusion on its perceived powers. Indeed, congressional touchiness has encouraged White Houses throughout U.S. history to think carefully about what powers to exercise.

Mr. Obama has not needed to think, carefully or otherwise. Name a prominent Democrat—name any Democrat—who has said boo about the president’s 23 unilateral rewrites of ObamaCare. Or of immigration law. Name any who today are defending constituents in their districts against the abuses of the Obama IRS. A few congressional Democrats got their backs up with the White House over possible Syria action, but they are dwarfed by the majority who’ve gone silent over Mr. Obama’s national-security policies—which they once berated George W. Bush for pursuing as an “imperial” president.

The main culprits here are Senate Majority Leader Harry Reid, and Ms. Pelosi, who’ve put themselves and their caucuses at the disposal of the White House. Winning political battles—sticking it to the GOP—is their priority, not constitutional balance. Mr. Reid has made himself White House gatekeeper, sitting on thorny votes, earning Congress public scorn for dysfunction. His members are meanwhile happy for Mr. Obama to pervert the law, since it saves them taking tough votes.

It hasn’t helped that much of the institutional memory of the Democratic Party has retired or died this past decade. Nearly half of today’s Democratic Senate was elected with or since Mr. Obama and has never known institutional leadership.

West Virginia Democrat Robert Byrd —onetime Senate majority leader and fierce defender of congressional power—would have laid down on train tracks to protest Mr. Obama’s recess appointments when the Senate was not in recess. The current Senate Democrats cheered the president on. It was left to Republicans and a unanimous Supreme Court on Thursday to restore the Senate’s constitutional rights.

Yet it is probably asking too much of Senate Democrats to protest the president’s diminution of their powers when they won’t protest Mr. Reid’s. Alaska’s Mark Begich has yet to have a vote on a single one of his amendments in six years. Louisiana’s Mary Landrieu now runs the “powerful” Senate Energy Committee, but as Mr. Reid has neutered committees, she has as much luck getting a vote on the Keystone XL pipeline as she does if she were running the Senate cafeteria. The majority leader last year stripped the Senate of filibuster powers. The Obama senators cheered the dismantling of their institutional power.

Mr. Boehner’s lawsuit was put down by some as a cynical attempt to rally his midterm voters. What this misses is that the Boehner lawsuit, if successful, would reassert the rights of all members of Congress—regardless of party, position or president in power. Democrats might consider thanking him for doing their job.

Write to kim@wsj.com