“Is this Phony Witch Hunt going to go on even longer so it wrongfully impacts the Mid-Term Elections, which is what the Democrats always intended? Republicans better get tough and smart before it is too late!” Trump tweeted.
He added that the Russia investigation is also “losing credibility.”
Trump’s Twitter comments come a day after his newly hired lawyer said the president may not cooperate with a subpoena from special counsel Mueller into the Russia probe and may plead the Fifth Amendment.
WATCH: Rudy Giuliani says Donald Trump could take the Fifth in Russia Russia investigation
In an appearance on ABC’s This Week on Sunday, Rudy Giuliani spoke about Trump and the investigation into Russia’s interference in the 2016 election.
In March, Mueller raised the prospect of issuing a grand jury subpoena for Trump in relation to the investigation. But Giuliani said despite Trump’s openness to sit down with Mueller, he would strongly advise the president against it.
“How can I ever be confident of that?” Giuliani said. “When I’m facing a situation with the president … in which every lawyer in America thinks he would be a fool to testify, I have a client who wants to testify.”
Giuliani said he wouldn’t rule out Trump asserting his Fifth Amendment right against self-incrimination in the Russia probe.
The Fifth Amendment to the U.S. Constitution states that no individual can be “compelled in any criminal case to be a witness against himself.”
WATCH: Trump says. ‘I would love to speak to Mueller, as long as I would be treated fairly’
“I’m going to walk him into a prosecution for perjury like Martha Stewart?” Giuliani asked, referring to the lifestyle maven convicted in 2004 of lying to investigators and obstruction in an insider trading case.
A subpoena fight would likely find its way to the Supreme Court, which has never firmly decided whether presidents can be compelled to speak under oath.
During Watergate, the court ruled that a president could be compelled to comply with a subpoena for tapes and documents. President Bill Clinton sat voluntarily with independent prosecutor Ken Starr after being issued a subpoena to discuss his relationship with former White House intern Monica Lewinsky, avoiding the constitutional question.
WATCH: Trump’s lawyers preparing for their client to be interviewed by Robert Mueller, report says
Trump previously slammed Fifth Amendment
During his run for the White House, Trump slammed Hillary Clinton staffers who invoked their Fifth Amendment rights when requested to testify during Clinton’s email scandal.
“The mob takes the Fifth,” Trump said. “If you’re innocent, why are you taking the Fifth Amendment?”
— With files from the Associated Press