Veteran investigative journalist Carl Bernstein said Republicans are privately questioning President Trump's sanity.

During an appearance on CNN, Bernstein said Republicans in the House and Senate who have shown a lack of interest in "confronting the abuse of power" may soon be changing their tune as Trump faces impeachment, exacerbated by controversies involving Ukraine and Syria.

"We have a situation now that there are three things happening at the same time with the president: He has undermined the national security of the United States by engaging a foreign power to subvert our American electoral process, he has gone into Turkey and endangered our national security there, and allowed the Russians once again, almost inexplicably, to be the winners," the Watergate sleuth said on Wednesday.

"He is behaving and conducting himself in a way that Republicans themselves, especially talking with reporters privately, are questioning his sanity and his stability. All this is happening at once. And at some point the Republicans are going to have to confront the fact that he has abused the power of the presidency, that there is a really strong case for high crimes here. And they’re going to have to confront that reality and look down the road about those Republicans who don’t confront it," he added.

Trump's critics have repeatedly questioned his mental stability, including George Conway, White House counselor Kellyanne Conway's husband. Last week, House Speaker Nancy Pelosi questioned Trump’s mental health after claiming he had a "meltdown" with Democratic congressional leaders. Trump responded by saying the California Democrat had an “unhinged meltdown.”

Trump, 73, received good health news after his annual physical in February. “While the reports and recommendations are still being finalized, I am happy to announce the President of the United States is in very good health and I anticipate he will remain so for the duration of his presidency, and beyond,” a statement from White House doctor Sean Conley said.