Watergate whistleblower John Dean: 'I still have trouble using the title Mr. President for someone installed by Putin'

John Dean, President Richard Nixon's former White House counsel, lamented on Sunday that he was not comfortable calling Donald Trump "Mr. President" because Trump was "installed" by Russian President Vladimir Putin.

In a tweet on Sunday, Trump referred to Dean as a "rat" for testifying against his Nixon in the Watergate hearings. The New York Times recently reported that current White House counsel Don McGahn has "extensively" coooperated with special counsel Robert Mueller's Russia investigation.

Dean began his response to Trump with a zinger.

"I still have trouble using the title Mr. President for someone installed by Putin," he quipped.

"I doubt you have ANY IDEA what McGahn has told Mueller," Dean added. "Also, Nixon knew I was meeting with prosecutors, b/c I told him. However, he didn’t think I would tell them the truth!"

Read Dean's tweet below.