Sen. Lindsey Graham, Republican of South Carolina and one of President Donald Trump’s closest congressional allies, has died at 71 following what his office described as a brief and sudden illness.

Graham’s death was confirmed by his Senate office. No further details on the cause were immediately released.

Trump paid tribute in a statement, calling Graham a “true American patriot” who would be “greatly missed.” The two men forged a tight political alliance during Trump’s first term after a rocky start in the 2016 primary contest.

Graham had represented South Carolina in the Senate since January 2003, previously serving four terms in the House. He held senior positions on the Armed Services Committee and the Judiciary Committee, and was a visible presence on foreign policy debates for more than two decades.

He is the second sitting senator to die in office this year.

Under South Carolina law, Gov. Henry McMaster will appoint an interim replacement to serve until a special election can be scheduled. McMaster’s office had not announced a timeline as of publication.