Opinion | House Democrats Can Constrain Trump. They Can Also Overreach.

After Donald Trump won the election in 2016, he was in a position of great power to cut taxes, repeal Obamacare and build border walls to his heart’s content — if congressional Republicans cooperated. Of course, they didn’t always cooperate, but Mr. Trump was also able to push forward his agenda using the presidency’s unilateral powers — to deregulate, start trade wars and expel undocumented immigrants — and his power to appoint conservative judges with the consent of the Senate.

That was then. When newly elected members of Congress take their seats in January, creating a Democratic majority in the House, the president’s power will shrink — not to nothing but significantly.

The reason is not that a hostile Congress will block legislation that Mr. Trump seeks. Even without party control of both houses, many presidents employed their unilateral powers and skillful deal-making to score political victories. Ronald Reagan cut taxes, weakened regulations and negotiated historic nuclear arms reduction agreements with the Soviet Union. Bill Clinton pursued his New Democrat vision with a largely Republican-dominated Congress. George W. Bush’s “war on terror” was almost entirely formulated by the executive branch. Barack Obama pursued several progressive measures — in the areas of immigration, affirmative action and campus speech — on his own.

Nor is Mr. Trump’s problem impeachment, at least, not in the short term. Removal requires a two-thirds vote in the Senate, and Republicans will almost certainly not oblige.

No, Mr. Trump’s power will shrivel for another reason: the interaction of congressional oversight and his chaotic, divisive and corrupt administration.

Congress has so far used its oversight powers reluctantly. Republicans want to protect Mr. Trump, not embarrass him. That will change with Democratic control of the House. Committees will hold hearings on potential election collusion with Russia, of course, but also Mr. Trump’s immigration enforcement policies, including family separation, his financial entanglements with the federal government, nepotism in the White House and allegations of corruption in the Trump Organization. Remember Whitewater? That single shady real estate transaction spawned a five-year investigation that consumed much of Mr. Clinton’s presidency. President Trump and the Trump Organization have (or so it has been alleged) been involved in dozens if not hundreds of shady transactions.

The parade of witnesses will be long. The president seems to expect his aides to corroborate the alternative reality he tweets out in the dead of morning, but they, not he, face perjury convictions if they do. Those witnesses will confirm accounts of dysfunction already leaked to the media, further damaging the president’s standing. Congress will demand to see Mr. Trump’s tax records and other financial documents. The bonanza of information will put him into jeopardy of further criminal investigation as well as civil lawsuits and, of course, bad press.

The investigations will also clog the gears of his already notoriously inefficient administration. The subpoenas alone will sap resources and damage morale. While Mr. Trump can still push agencies to deregulate, hold the line on immigration enforcement, raise tariffs and tear up treaties, every action will invite a new congressional inquiry, along with a blizzard of subpoenas and another parade of witnesses if — as so often seems the case — it is accompanied by leaks that suggest procedural irregularity, internal disagreement or impropriety.

The White House will fight back by claiming executive privilege. This hazy area of the law provides many opportunities for delay, confusion and conflict. But executive privilege cannot be used to disrupt investigations into criminal matters, and most presidents have felt constrained to allow inquiries into serious allegations of unethical conduct.

Will Mr. Trump? He could order his subordinates not to answer subpoenas. He could certainly get them to drag their feet. And while Congress could refer those officials to the Justice Department for prosecution for contempt, the department’s longstanding policy is not to prosecute officials who keep silent based on executive privilege. But Congress can go directly to the courts to obtain a declaration that an official is in contempt, and — in extremis — arrest and detain him itself. This won’t happen, but expect some tense moments.

With or without executive-branch cooperation, Congress will uncover suspicious activities, which it will refer to the Justice Department. Attorney General Jeff Sessions (or his successor) will be pressured to appoint special counsels. These new Robert Muellers could generate new clashes with the president, whose distrust of the Justice Department will reach new heights. If Mr. Trump survives these conflicts, he will be badly wounded.

Unless the Democrats rescue him. A premature impeachment inquiry could do just that. Bill Clinton’s popularity soared after the Republicans’ attempt to remove him from office blew up in their faces in 1999. No president has been pushed out of office except Richard Nixon, whose poll numbers languished in the 20s for months before his resignation and who finally lost the support of leading Republicans as well. Mr. Trump’s numbers have stayed near or above 40 percent for most of his tenure, and he has a stranglehold on the Republican Party.

In existential contests between Congress and the presidency, Congress can’t win without overwhelming public support. The presidency enjoys too many inherent advantages over a clumsy multimember body that is deeply unpopular and divided with conflicts. Mr. Trump can’t be defeated, not until 2020. But if Congress acts wisely, he can be constrained.

Eric Posner is a professor at the University of Chicago Law School.

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