CHICAGO (CBS) — After President Donald Trump warned fired FBI Director James Comey about “leaking to the press” about their conversations, U.S. Sen. Dick Durbin said the president’s tweets “border on intimidation.”

“James Comey better hope that there are no ‘tapes’ of our conversations before he starts leaking to the press!” the president tweeted Friday morning.

Although it was unclear what prompted Trump’s tweet, the New York Times reported hours earlier that the president had asked for Comey’s loyalty when the two had a private dinner in January at the White House. According to the New York Times, two people who have heard his account of the dinner said Comey declined to make that pledge, and now believes that conversation “was a harbinger of his downfall.”

Trump fired Comey Tuesday evening. The White House has said Comey was fired for his handling of the Hillary Clinton email investigation, but Trump has said he considered “this Russia thing” when he decided to fire Comey.

Durbin said Trump’s words since firing Comey raise questions about whether the president is trying to interfere with the FBI’s investigation of Russia’s meddling with the election, and possible ties between Russia and the Trump campaign.

“President Trump is in dangerous territory. What we are facing now is a question of whether there’s obstruction of justice,” he said. “The president, with his tweets this morning, has said things that border on intimidation against Comey and any others who are going to disagree with him publicly on this issue. That is a serious matter, and it’s dangerous when it comes to our constitution.”

Durbin and other Illinois Congressional Democrats don’t want to wait for a new FBI director to be appointed; they want a special prosecutor to be appointed now to investigate allegations of Russian interference with the election and collusion with the Trump campaign.

“This just isn’t another wild tweet by the president. He’s going into territory with obstruction of justice. It’s very serious,” he said.

U.S. Rep. Jan Schakowsky said Comey is the third person who was investigating Trump to be fired – including acting Attorney General Sally Yates and New York U.S. Atty. Preet Bharara.

“The president’s action begs the question what exactly is he hiding? We can’t trust this administration to investigate itself. We need an independent investigation free from influence of the Trump attorney general and other administration officials,” she said.

Democrats do not have the votes to force the appointment of a special prosecutor by the House or Senate.

“What we can hope is that a number of Republicans will step up for the good of this nation, and agree that a special prosecutor is necessary,” he said.

The White House has said a special prosecutor is not necessary, because there are enough investigations underway already.

“It’s simply not necessary. You’ve got multiple groups already looking into this: the House committees, the Senate committees, the deputy attorney general through the FBI. They are already looking at this, I don’t think that it is necessary to bring in another individual to take this on,” Sanders told host Charlie Rose on “CBS This Morning” on Thursday.

Durbin said Deputy Attorney General Rod Rosenstein – who wrote a letter to Trump recommending Comey be fired – should agree to a special prosecutor, or step down.

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