While Mr. Barr did not use the word “impeachment” in his speech, he laid out a new defense of Mr. Trump that was taken up by Republicans on Capitol Hill. In an effort to invalidate the inquiry, lawmakers had argued that the president did not withhold a White House meeting or military aid to pressure President Volodymyr Zelensky of Ukraine to publicly announce investigations that would benefit Mr. Trump politically.

After a week of damaging public hearings in which multiple witnesses offered new details of the president’s pressure campaign and said that he spoke openly of his desire that Ukraine publicly announce investigations, Mr. Trump’s supporters began to argue that he had acted within his rights.

Mr. Trump has also begun to echo Mr. Barr’s assertions. In an interview on Fox on Friday, he said that the decision to investigate his 2016 campaign’s ties to Russia “was an overthrow attempt at the presidency.”

Now that the claim that Mr. Trump never pressured Mr. Zelensky no longer holds, “the argument has got to be a ‘so what’ argument — Bill Barr’s argument that the president did all these things, but this is what a president can do,” said Stuart Gerson, a Checks & Balances member who was a senior campaign adviser to George Bush and a Justice Department official in his administration.

“The Republicans in the Senate and in the House think they’re in a Parliament, and their responsibility is to a prime minister to whom they owe party loyalty,” Mr. Gerson said. “That’s not the American tradition. One can recognize substantial executive power, but that doesn’t mean the legislative branch should be dead.”

Mr. Barr has argued that his view of presidential power stems directly from the Constitution. It delineates the responsibilities of the three branches of government, he has said, rather than allowing the legislature and the judiciary to check the powers of the president as two of three co-equal governing powers.

That interpretation of history “has no factual basis,” Checks & Balances wrote in its statement, including the claim that “the founders shared in any respect his vision of an unchecked president, and his assertion that this view was dominant until it came under attack from courts and Congress a few decades ago.”