Paul Krugman, the Nobel Prize winning economist and contributor at The New York Times, tweeted Tuesday that President Trump is “obviously mentally ill.”

“An American first: a president who was obviously mentally ill the moment he took office. Thanks Comey,” the tweet read.

He was apparently referring to FBI Director James Comey who some Democrats still blame for informing Congress-- 11 days before the election—that he was reopening the investigation into Hillary Clinton’s email server.

An American first: a president who was obviously mentally ill the moment he took office. Thanks, Comey https://t.co/FuZbCy5DxQ — Paul Krugman (@paulkrugman) January 24, 2017

Krugman has been a vocal critic of Trump and his policy proposals. He also recently defended Rep. John Lewis’ decision not to attend the inauguration of Trump.

He asks in the column if Lewis is right to declare “the man about to move into the White House illegitimate?” He wrote, “Yes, it is. In fact, it’s an act of patriotism."

He goes on, “No, we shouldn’t get into the habit of delegitimizing election results we don’t like. But this time really is exceptional, and needs to be treated that way.”