Conservative author Jerome Corsi, who has refused to sign a plea deal with special counsel Robert Mueller, on December 3 filed a "criminal and ethics complaint" against Mueller's team.
The president praised another longtime adviser, Roger Stone, for saying he would not testify against the president.
"Those who said the president's warm and complimentary tweet constitutes witness tampering should be reminded that I have never been contacted by any investigative body and therefore, by definition, this can not be true", Stone told the Daily Caller's Saagar Enjeti.
Mueller has negotiated cooperation deals with several key witnesses, notably Trump's former lawyer Michael Cohen, in exchange for reduced prison sentences for various charges.
President Trump at the G-20 summit in Buenos Aires. "I've had no discussion regarding a pardon", Stone said. "Nice to know that some people still have 'guts!'"
Trump followed up his attacks on Cohen with yet another slam on Mueller, whose investigators secured the additional guilty plea Friday and allowed him to reach the cooperation agreement.More news: Modric wins Ballon d'Or to break Messi-Ronaldo dominance
Stone also denied having had any contact during the 2016 presidential campaign with Julian Assange, the creator of WikiLeaks, the site that published hacked messages from Trump's Democratic opponents in an apparent bid to influence the election.
Cohen told a federal judge that negotiations for a Trump Tower real estate project in Moscow were still going on as late as June 2016, just one month before Trump earned the Republican Party's nomination.
Rep. Adam Schiff said Sunday on ABC's "This Week" that emails between Stone and an associate, Jerome Corsi, are "inconsistent" with Stone's testimony before the congressional committee.
George Conway, the husband of White House counselor Kellyanne Conway, appeared Monday to accuse Donald Trump of witness tampering and obstruction of justice after the president praised Roger Stone for refusing to testify against him.
Stone himself had an answer to the suggestion that Trump's tweet may have violated witness tampering statutes. "The truth is very bad for their mission!"