WASHINGTON — As a lawyer in private practice, Pat A. Cipollone, now President Trump’s White House counsel, told colleagues that there were two approaches to legal fights.

One, he said, was like the Department of State, when the two sides would try to work out a deal to avoid painful and expensive litigation. The other, when the first failed, was the Department of War.

As of this week, Mr. Cipollone has put himself squarely in the war camp when it comes to Mr. Trump’s defense against the House impeachment inquiry. After earlier advocating that the president adopt a policy of transparency by releasing the document at the heart of the impeachment debate — the reconstructed transcript of Mr. Trump’s call with his Ukrainian counterpart — Mr. Cipollone has shifted course and is now leading a no-cooperation strategy that holds substantial political risks but also seems to suit his combative client in the Oval Office.

It is a role that has pushed Mr. Cipollone, 53, to the center of a battle that could determine the course of Mr. Trump’s presidency and potentially lead to a constitutional battle with far-reaching ramifications. In building an argument that Mr. Trump has no obligation to respond to demands for information from Congress, Mr. Cipollone, in a letter sent Tuesday to House Democratic leaders, laid out an extraordinarily broad view of executive authority that, if maintained, seems likely to be viewed skeptically by the courts.