Story highlights "I don't know what he'll find, but I trust him," the Democratic senator says of the special counsel

Mueller has been appointed to investigate Russia's influence on the 2016 presidential election

(CNN) Deomcratic Sen. Dick Durbin said Friday that President Donald Trump will put the country at the "brink of a constitutional crisis" if he fires special counsel Robert Mueller.

Newsmax CEO Chris Ruddy, a friend of the President, said earlier this week that Trump was considering firing Mueller, who has been appointed to investigate Russia's influence on the 2016 presidential election. On Friday, the President tweeted he was under "investigation," which he called a " witch hunt " in another tweet.

Durbin, speaking on WGN radio, said he hoped the response to Trump firing Mueller would be bipartisan if it occurred.

"If he fires Mueller, I will tell you, we are right on the brink of a constitutional crisis," the Illinois senator said. "Mueller is an independent counsel, well-respected man, doing a good job. I don't know what he'll find, but I trust him -- trust him to put our country first. If the President tries to fire him to diminish or stop this investigation, I would hope that finally there would be a bipartisan uproar against it."