WASHINGTON — President Donald Trump praised a key witness in the Russia investigation Monday for having the “guts” not to testify against him, and said his former lawyer — who cut a deal with prosecutors — should head straight to prison.

In a pair of politically charged tweets, Trump made clear that he is closely watching those who turn on him in the special counsel’s probe, which has ensnared some of the president’s closest advisers. So far, five people in Trump’s orbit have pleaded guilty to federal charges.

The tweets add to mounting questions about whether Trump is taking steps to improperly influence witnesses in an investigation that has enraged him and shadowed his administration. Some legal experts, though, say they may not amount to witness tampering if Trump didn’t directly tell others what to say or not say.

Trump already has come under scrutiny from critics who fear he may use his executive power to protect himself as well as friends and supporters. Last week, Trump told the New York Post that a pardon for his former campaign chairman Paul Manafort was not off the table.

Prosecutors say Manafort torpedoed his plea deal with special counsel Robert Mueller by repeatedly lying to them, although Manafort denies that he lied.

In one of Monday’s tweets, Trump took aim at Michael Cohen, his former personal attorney who once grandly declared he would “take a bullet” for the president but ultimately took a plea deal.

Cohen pleaded guilty last week to lying to Congress about negotiations he had on Trump’s behalf for a real estate deal in Moscow.

Though he told lawmakers the talks were done by January 2016, he admitted they actually lasted as late as June — after Trump had clinched the Republican nomination and after Russians had penetrated Democratic email accounts for communications later released through WikiLeaks.

Cohen said he lied out of loyalty to Trump, who insisted throughout the campaign that he had no business dealings in Russia, and to be consistent with his political messaging.

On Monday, Trump ripped into Cohen on Twitter.

“You mean he can do all of the TERRIBLE, unrelated to Trump, things having to do with fraud, big loans, Taxis, etc., and not serve a long prison term?” Trump added that Cohen “makes up stories to get a GREAT & ALREADY reduced deal for himself.”

“Michael Cohen asks judge for no Prison Time.” You mean he can do all of the TERRIBLE, unrelated to Trump, things having to do with fraud, big loans, Taxis, etc., and not serve a long prison term? He makes up stories to get a GREAT & ALREADY reduced deal for himself, and get….. — Donald J. Trump (@realDonaldTrump) December 3, 2018

Trump added: “He lied for this outcome and should, in my opinion, serve a full and complete sentence.”

Minutes later, Trump lavished praise on his former campaign adviser Roger Stone. Mueller’s prosecutors are investigating Stone to learn whether he had advance knowledge of WikiLeaks’ plans to release hacked material damaging to Hillary Clinton’s presidential effort.

Trump lauded Stone for saying he’d never testify against the president.

“This statement was recently made by Roger Stone, essentially stating that he will not be forced by a rogue and out of control prosecutor to make up lies and stories about ‘President Trump,'” he tweeted. “Nice to know that some people still have ‘guts!'”

“I will never testify against Trump.” This statement was recently made by Roger Stone, essentially stating that he will not be forced by a rogue and out of control prosecutor to make up lies and stories about “President Trump.” Nice to know that some people still have “guts!” — Donald J. Trump (@realDonaldTrump) December 3, 2018

Stone then posted a screenshot of Trump’s tweet with a caption that said he was proud of their 40-year relationship and “prouder still of the amazing job he is doing making America Great Again!”

Sen. Mark Warner, the top Democrat on the Senate intelligence committee, said Trump’s tweet was inappropriate.

“The President of the United States should not be using his platform to influence potential witnesses in a federal investigation involving his campaign,” Warner said in a tweet.

Stone said the idea that Trump’s tweet amount to witness tampering is “hysterical.”

“I’m not a witness to any proceeding,” he said.

David Weinstein, a former Justice Department prosecutor in Florida, said he was surprised by Trump’s comments Monday, but didn’t believe the tweets alone rose to the level of obstruction or witness tampering because Trump did not explicitly tell anyone what to say or not to say. Subjects of an investigation can still communicate to others entangled in a probe, and though they can encourage them to tell the truth, they cannot coach them to lie, he said.

“What he seems to be saying is that people who continue to show support for him, in some way, may be rewarded for that support,” Weinstein said. “I don’t think it rises to the level of obstruction yet, but it certainly would cause people who are conducting the investigation to start asking questions about whether or not the target has reached out to them.”

Trump’s message had an immediate effect on supporters. His remarks prompted Michael Caputo, the president’s former campaign aide and a longtime Stone friend, to launch a “GoFundMe” account to help pay Stone’s mounting legal fees.

Stone said he’s paid about half a million in legal fees already and is projecting that total to reach $2 million.

“I require a small platoon of excellent lawyers and they’re not inexpensive,” he said.

The Russia investigation has dogged Trump for two years.

A conservative author, who is an associate of Stone and in the crosshairs of Mueller’s investigation, filed a complaint Monday with the Justice Department, alleging prosecutors tried to coerce him to give false testimony and threatened to indict him.

Investigators are looking into whether Jerome Corsi had contact with WikiLeaks or knew about their plans to release emails damaging to Clinton. Corsi has released documents showing Mueller’s prosecutors offered him a deal to plead guilty to a false statements charge but he’s rejected the offer. Corsi says he didn’t knowingly mislead investigators and wasn’t in contact with WikiLeaks. The Justice Department declined to comment on his complaint.

The Russia investigation has dogged Trump for two years. In recent weeks, Trump has sharpened his criticism, accusing Mueller’s prosecutors of dirty tactics and pressuring witnesses to lie.

Cohen’s decision to turn on his former boss was a particularly striking blow for the president.

Cohen pleaded guilty in August to eight criminal counts, including campaign-finance violations, in a separate case unrelated to Mueller’s investigation. He said Trump directed him to arrange the payment of hush money to porn star Stormy Daniels and a former Playboy model in the run-up to the 2016 campaign.

Cohen’s admission during that court appearance marked the first time that a Trump associate had gone to court and implicated Trump in a crime.

Whether — or when — a president can be prosecuted remains a matter of legal dispute. Trump has denied any wrongdoing as well as the extramarital affairs.

Associated Press writers Chad Day and Jill Colvin contributed to this report.

READ MORE: The giant timeline of everything Russia, Trump and the investigations