George Conway, the husband of White House Senior Adviser Kellyanne Conway, on Wednesday called U.S. President Donald Trump “the most unfit and incompetent president in American history.”

Conway was responding to an early Wednesday morning tweet in which Trump claimed “Without the ILLEGAL Witch Hunt, my poll numbers, especially because of our historically 'great' economy, would be at 65%. Too bad!”

Conway responded, “Without your bizarre, improper, impeachable, and ILLEGAL behavior in response to the perfectly justified investigation you like to call a 'Witch Hunt,' and without your deranged behavior in numerous other respects … your poll numbers, especially because of the great, full-employment economy that you inherited and then goosed up with massive deficit spending, would be much higher than they are. Too bad! The most unfit and incompetent president in American history.”

The Twitter exchange came as Democrats in the U.S. Congress debated possibly impeaching Trump, House Speaker Nancy Pelosi said on Wednesday, about an hour before a White House meeting with him, that Trump is engaged in a "cover-up."

The president is stonewalling multiple congressional investigations by ignoring subpoenas, refusing to allow current and former advisers to testify and not handing over documents, steps that have aggravated a confrontation with Congress.

"No one is above the law, including the president of the United States. And we believe that the president of the United States is engaged in a cover-up," Pelosi told reporters after a morning meeting of House of Representatives Democrats.

She and other congressional leaders were scheduled to meet mid-morning at the White House with Trump to talk about a potential bipartisan infrastructure development plan, although a firm proposal for funding any such effort has yet to emerge.

It was unclear if the meeting would occur after Trump wrote on Twitter about the time it was to begin: "As I have long been saying, and has now been proven out, this is a Witch Hunt against the Republican Party and myself, and it was the other side that caused the problem, not us!"

Trump and Democrats who control the House are engaged in a high-stakes power struggle over their ability to investigate him, with the president increasingly asserting that his advisers need not respond to lawmakers' inquiries.