U.S. Ambassador William Hagerty will leave his post to run for the Senate, President Donald Trump tweeted late Friday from Washington, garnering a “Complete & Total Endorsement” from the American leader.

“Tennessee loving Bill Hagerty, who was my Tennessee Victo(r)y Chair and is now the very outstanding Ambassador to Japan, will be running for the U.S. Senate,” Trump tweeted. “He is strong on crime, borders & our 2nd A. Loves our Military & our Vets. Has my Complete & Total Endorsement!”

The announcement, which could not immediately be confirmed by Hagerty’s office, comes at a time when Japan, a top U.S. ally, is embroiled in a heated trade and history-related row with South Korea, another key American partner in the region.

His departure from the high-profile post would leave yet another vacancy in an administration that is struggling to fill important diplomatic — and even Cabinet-level — positions.

The United States has traditionally played a role as a mediator in disputes between the two neighbors, and it is unclear how any decision to leave his post would affect that.

A member of Trump’s transition team, Hagerty was sworn in as ambassador to Japan nearly two years ago. He served as commissioner of economic development for Tennessee from 2011 to 2015.

The ambassador has a history with Japan, having been posted to Tokyo for three years from the late 1980s to early 1990s while working for the Boston Consulting Group.

Hagerty helped convey the Trump administration’s position on its ongoing diplomatic entreaties with nuclear-armed North Korea, as well as bolstering the allies’ defense relationship.

He also helped in facilitating talks on a bilateral trade deal with a reluctant Japan.

Hagerty told The Japan Times in an exclusive interview last month that Trump is “frustrated” by the fact that the U.S. and Japan have yet to ink a trade deal, which was reflected in his recent mention of August as a desirable deadline.

Hagerty will seek the seat long-held by Republican Sen. Lamar Alexander, who announced in December he would not seek re-election.

Alexander told CNN that Hagerty would benefit from his “close” relationship with Trump in any primary race. The report also said the ambassador maintains a positive relationship with Trump’s daughter and adviser Ivanka.

However, he must resign from his ambassadorship before he can officially throw his hat into the U.S. Senate race, a process that is reportedly under way. Media reports have said that Ward Baker, who ran Republican Sen. Marsha Blackburn’s campaign last year, is expected to head up Hagerty’s team.

James Schoff, a former senior Pentagon East Asia specialist now with the Carnegie Endowment for International Peace in Washington, said Hagerty had “served pretty well and stayed “on Trump’s good side” — something he said was “not easy.”

In terms of finding a replacement, Schoff said it “could take till the rest of the year, unless they’ve been setting this up for a while.” While he said that was possible, he said he expected the confirmation process to take “a decent amount of time.”