After striking an increasingly combative tone following the release of Mr. Mueller’s findings, Mr. Trump proclaimed on Wednesday that the White House would be “fighting all the subpoenas” because he considered them partisan and politically motivated.

“These aren’t, like, impartial people,” the president said.

In an interview on Tuesday with The Washington Post, the president characterized the demands as simply the work of one party, suggesting that was sufficient reason to ignore them even though they represent official requests from the House, not just House Democrats. “I don’t want people testifying to a party, because that is what they’re doing if they do this,” Mr. Trump told The Post.

The Justice Department backed up his stance. It disclosed that Attorney General William P. Barr would prohibit John Gore, a top official in the agency, from appearing Thursday as ordered to discuss any role in adding a citizenship question to the 2020 census despite the fact that he was called under a bipartisan subpoena.

That move came on top of the administration’s position that it did not want Donald F. McGahn II, the former White House counsel, to testify about the Mueller report, as well as its instruction to Carl Kline, the former White House personnel security director, not to appear to answer questions about the issuance of security clearances. The Treasury Department also refused to obey an order from the Ways and Means Committee to produce the president’s tax returns despite a law that gives the panel the authority to obtain the documents.

“This is a massive, unprecedented and growing pattern of obstruction,” said Representative Elijah E. Cummings, Democrat of Maryland and the chairman of the oversight panel. He also warned administration officials to be careful in blindly following the president’s instructions.