Stephen I. Miran may be filling only a temporary opening on the Federal Reserve’s powerful board of governors, but in that role he can influence important discussions in and around the central bank, which President Trump has repeatedly attacked as an obstacle to his economic agenda.

Officials at the Fed are deliberating when and by how much to lower interest rates after a long pause, and there is a tight contest underway for who will be named the next chair of the institution.

Mr. Miran, the chair of Mr. Trump’s Council of Economic Advisers, was tapped on Thursday to serve as a governor for a term that expires on Jan. 31. That position was formerly held by Adriana Kugler, who officially stepped down on Friday after abruptly announcing last week that she would leave the post early and return to academia.

Her departure gave Mr. Trump a way to directly alter the upper echelons of the Fed, which has emerged as one of the president’s biggest foes since his return to the White House. Mr. Trump wants the Fed to cut interest rates significantly, but the central bank has opted to keep them steady this year, anticipating that the Trump administration’s tariff and other economic policies would unleash higher inflation even as growth slows.

That has made the Fed’s chair, Jerome H. Powell, a frequent target of attacks by Mr. Trump, who has long sought to shake up the central bank since naming Mr. Powell as chair in his first term.

An Insider to ‘Trumpet the President’s Message’

Once confirmed by the Senate — a process that could take months — Mr. Miran would hold one of the most powerful positions at an institution he has staunchly criticized in the past.