"An American first: a president who was obviously mentally ill the moment he took office," Krugman tweeted along with a New York Times report that Trump brought up debunked claims that millions of people voted illegally in the election, causing him to lose the popular vote, during a Monday night meeting with Congressional leaders.

"Thanks, Comey," he added, referring to FBI Director James Comey.

Krugman has criticized Trump repeatedly since his election — questioning Trump's business conflicts of interest, his Cabinet nominees, the involvement of Russia in his election, and more. But this is the first time Krugman called into question the mental fitness of the president.

Trump received a clean bill of health from his doctor before the election.

Krugman has also been critical of Comey, arguing that Comey's letter to Congress regarding the renewed investigation into Hillary Clinton's use of a private email server swung the election in Trump's favor.

Krugman highlighted part of a Washington Post article about Trump's attitude after the election. "Read it and shudder," he wrote.

In subsequent tweets, Krugman said that despite Trump's best efforts, the growing difference between increasing US interest rates due to rate hikes from the Federal Reserve and the interest rates throughout the rest of the world would increase the strength of the US dollar. A strong dollar would be harmful to US manufacturing, offsetting any pro-manufacturing policies from Trump, Krugman said.

Here are the tweets: