WASHINGTON — House Democrats’ feud with Attorney General William P. Barr boiled over on Thursday, as Speaker Nancy Pelosi accused him of lying to Congress and the Judiciary Committee threatened to hold him in contempt if he did not promptly hand over a complete version of Robert S. Mueller III’s report.

The escalation between the legislative and executive branches of government, a day after Mr. Barr, the nation’s top law enforcement officer, mounted an aggressive self-defense in the Senate, was as abrupt and emotionally charged as any in decades.

The Justice Department had ignored a Wednesday deadline to provide an unredacted version of the report by Mr. Mueller, the special counsel, and the investigative materials used to compile it. Then, on Thursday morning, Mr. Barr failed to appear at a House hearing on Mr. Mueller’s investigation of Russian election interference, and possible obstruction of justice by President Trump, because of a dispute over who would be allowed to question the attorney general.

But it was a newly revealed letter from Mr. Mueller to the attorney general that most provoked Ms. Pelosi’s ire.