“We’re fighting all the subpoenas.”

With this vow to reporters on Wednesday , President Trump laid bare his approach to the concept of congressional oversight. As the Democratic-controlled House ramps up its investigations of his administration, Mr. Trump is throwing up a stone wall.

Among the congressional requests denied in recent days: Mr. Trump’s top immigration adviser, Stephen Miller, has declined an invitation to discuss immigration policy; the Justice Department has rebuffed a subpoena for John Gore, a deputy attorney general, to answer questions about the 2020 census; and the White House has instructed a former staffer, Carl Kline, not to provide information about how security clearances are granted. Mr. Trump called the subpoena for Donald McGahn, the former White House counsel, “ridiculous.” And while the Internal Revenue Service is still officially considering the request by the House Ways and Means Committee for Mr. Trump’s tax returns, the president’s acting chief of staff has already declared that Democrats will “never” get their hands on them.

Mr. Trump and his defenders justify such defiance by saying that the inquiries are partisan and thus illegitimate. “These aren’t, like, impartial people,” the president said of Democratic lawmakers.

Mr. Trump is leaning heavily on executive privilege, the principle that the president and other senior officials have the right to confidential deliberations within the executive branch. But scholars say executive privilege is a tradition, not a law. Neither the phrase nor the concept appears in the Constitution. Even so, since the establishment of the presidency, all of its occupants have at some point claimed the implied prerogative as fundamental to the separation of powers.