Donald Trump is said to have become increasingly worried about the prospect of being impeached in recent days, as Robert Mueller's Russia investigation inches closer to the president.

Despite a public declaration that he is not concerned about the threat of impeachment, the progress of the Mueller probe and federal investigations into his former associates, coupled with Democrats taking over the House of Representatives, has left the president alarmed.

The most alarming development, according to sources speaking to NBC News, came on Wednesday when federal prosecutors in New York announced they had reached an agreement with American Media Inc (AMI). The publisher of tabloid the National Enquirer admitted to making a $150,000 payment in 2016, in coordination with Mr Trump’s presidential campaign, to silence Karen McDougal who alleged an affair with Mr Trump. AMI have said it was to prevent the story influencing the presidential election.

AMI’s announced cooperation with prosecutors come on the heels of Michael Cohen, Mr Trump’s former personal attorney, pleading guilty to violating campaign finance laws in 2016 by arranging a hush money payment to adult actress Stormy Daniels over another alleged affair. That is as well as facilitating the payment to Ms McDougal. Mr Trump has denied the affair.

In a few weeks, Trump Organisation Chief Financial Officer Allen Weisselberg, who was allegedly and David Peck, the chief executive at AMI, will give testimony on the payment. Mr Weisselberg was granted immunity in exchange for his testimony.

“The entire question about whether the president committed an impeachable offense now hinges on the testimony of two men: David Pecker and Allen Weisselberg, both cooperating witnesses in the SDNY investigation,” an anonymous source, who is a close ally for Mr Trump, told NBC News.

The deal with AMI makes Mr Trump the only individual who is still arguing that those payments were not made to influence the election.

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The president, who is also scambling to find a new chief of staff, has reportedly been scrambling to seek advice and vent to his friends outside the White House and on Capitol Hill. The president reportedly was not in the Oval Office on Wednesday until noon.

The president is also said to have been frustrated at the lack of progress in getting enough funds to build a border wall. Mr Trump has also reportedly snapped at his own staff and Capitol Hill legislators.

But in the public’s eye, Mr Trump seems unfazed by the federal investigations and the likelihood of his impeachment. In an interview with Reuters, Mr Trump said he was not concerned with the prospect and that Americans would revolt if it had happened.

“It’s hard to impeach somebody who has'nt done anything wrong and who’s created the greatest economy in the history of our country,” Mr Trump said. “I’m not concerned, no. I think that the people would revolt if that happened.”

The White House did not respond to our request for comment.