With Watergate, those two conditions were met. When President Nixon refused to comply with a valid subpoena issued by a federal court, there was no clear answer in the Constitution as to which branch of government would prevail. Mr. Nixon wouldn’t budge, and neither the special prosecutor nor the court was willing to back down. The crisis was resolved only after the Supreme Court ordered the tapes to be turned over and Mr. Nixon resigned.

President Trump’s stonewalling of the House impeachment inquiry also satisfies the two conditions for a constitutional crisis. First, the Constitution doesn’t indicate what is supposed to happen if the House tries to exercise its constitutional power of oversight to investigate the president and the president flatly rejects the House’s constitutional authority. Congress can demand that the president comply, but it can’t very well send its sergeant-at-arms to the White House to enforce its subpoenas.

You might say that under the Constitution, the House could justifiably impeach the president for refusing to participate in the impeachment inquiry. Indeed, the third article of impeachment adopted by the House Judiciary Committee against Mr. Nixon charged him with contempt of Congress for ignoring subpoenas issued by Congress.

Yet as Mr. Trump appears to be calculating, it’s not so simple or satisfying for the House to proceed to impeachment without a factual inquiry. Impeaching a president for refusing to participate in an impeachment inquiry is a kind of meta-impeachment. It would allow Mr. Trump to argue that the meta-impeachment is illegitimate because it isn’t based on an investigation.

That brings us to the second condition, namely that neither of the key actors seems prepared to back down. About the only thing the House could do now would be to pass a resolution formally authorizing the impeachment inquiry — something it has not yet done and which the White House counsel cited in his letter as a reason to consider the current inquiry “unprecedented.” Passing such a resolution might be a good idea for the House. But it would not qualify as backing down. To the contrary, passing a resolution to investigate impeachment would raise the stakes in the constitutional confrontation.