The Justice Department’s Office of Legal Counsel has traditionally been a powerful guardrail in American government.

It has issued interpretations of the law that bind agencies across the executive branch, decided which proposed policies were legally permissible or out of bounds and approved draft executive orders before they went to presidents to be signed.

But in President Trump’s second term so far, the office has largely been sidelined. As Mr. Trump issues policy after policy pushing legal limits and asserting an expansive view of his power, the White House has undercut its role as a gatekeeper — delaying giving it senior leadership and weakening its ability to impose quality control over executive orders.

Its diminished voice is shifting the balance of legal power in the executive branch toward the White House, speeding up Mr. Trump’s ability to act but creating mounting difficulties for the Justice Department lawyers who must defend the government in court.

“The Trump administration has cut out the traditional check the Justice Department has played,” said Jack Goldsmith, a Harvard law professor who led the office under the George W. Bush administration. “O.L.C. being cut out is a piece of a larger strategy designed to ensure that the president can do whatever he wants, without any internal executive branch legal constraints on his will.”

Two and a half months into his administration, Mr. Trump has neither submitted a nominee to be the office’s Senate-confirmed leader nor named an acting assistant attorney general. This week, after being asked to comment for this article, the department announced that he intended to nominate T. Elliot Gaiser, 35, the Ohio solicitor general, for the position.