The showdown that is brewing between Attorney General William Barr and the House Judiciary Committee poses enormous dangers for President Trump. It might not look that way at first blush. The feud, after all, is ostensibly over the terms under which Mr. Barr is to testify before the House about the report from the special prosecutor, Robert Mueller. It has yet to ripen into a formal impeachment proceeding.

Mr. Barr might be able to win this fight. He is merely resisting the idea that he is to be questioned not only by the honorable congresspersons but also by lawyers for the committee. It looks to us like he should never have agreed to share the Mueller report with the Congress in the first place. It’s an internal executive branch document. Even so he might be able to prevail against the House.

Particularly because of the Holder precedent. Mr. Holder is the only attorney general in history to have defied a congressional subpoena and to be found in contempt. Not just any nickel-plated civil contempt but criminal contempt of Congress. President Obama’s Justice Department balked at prosecution, and Mr. Holder walked. He claims to have been embarrassed, but he’s a free man.

Messrs. Trump and Barr might be able to get away with that. We’re not inclined to draw a lot of distinctions between the goose and the gander. If what’s going on in the House ripens into an impeachment investigation, though, all bets are off. That’s because of distinctions that have been drawn in our republic all the way back to Presidents George Washington and James Polk.

Both Washington and Polk acknowledged that once an impeachment process is started, the House becomes essentially irresistible. A study in 2001 by the Congressional Research Service recalls the fight over the Jay Treaty that President Washington struck with Britain. Washington denied the House access to his papers, saying they could be legitimately requested only in a case of impeachment.

President Polk, the CRS reported, went further. He said that in an impeachment, “the power of the House” would “penetrate into the most secret recesses of the Executive Department. It could command the attendance of any and every agent of the Government, and compel them to produce all papers, public or private, official or unofficial, and to testify on oath to all facts within their knowledge.”

Plus, too, mark this: Refusal to comply with a subpoena is the charge that was laid against President Nixon in the third of the articles of impeachment voted out by the Judiciary Committee in 1974. Article Three charged that Nixon had “failed without lawful cause or excuse to produce papers and things as directed by duly authorized subpoenas issued by the Committee on the Judiciary.”

In the case of Nixon, the House never sent articles of impeachment to the Senate. Instead, Republicans in the Senate panicked and sent a delegation to the White House to tell Nixon they were going to vote against him. It was a shocking default by weak senators. It would be as if jurors, acting before charges were handed up, told the suspect to plead guilty. It did, though, precipitate Nixon’s resignation.

We’re not suggesting Mr. Trump should be impeached. He has been cleared of the underlying charge of colluding with the Russ camarilla. The obstruction being investigated is, at best, ambiguous. That doesn’t mean, though, that the president is out of danger. Once the House moves to a formal impeachment proceeding, the constitutional afterburners kick in and Mr. Trump could easily be bound over to a trial in the Senate.