Speculation about the impeachment of Donald Trump is escalating in Washington, after the president’s former campaign chief Paul Manafort’s decision to cooperate with investigators.

Beautiful Country Burn Again review: Trump, 2016 and America's 'long con' Read more

Manafort pleaded guilty to two criminal charges on Friday morning and struck a plea deal agreeing to assist special counsel Robert Mueller’s inquiry into Russian interference in the 2016 election. The plea agreement set out how Manafort must turn over documents and brief officials about “his participation in and knowledge of all criminal activities”.

The Sunday politics shows were duly dominated by talk of growing peril for Trump following Manafort’s dramatic moment in court.

Adam Schiff, the leading Democrat on the House intelligence committee, told NBC’s Meet the Press: “Manafort is at the confluence of a number of pernicious interests. You’ve got the president’s son trying to get dirt from the Russians in Trump Tower, you’ve got the president himself asking the Russians for dirt on Hillary Clinton in a public statement.

Manafort is a key person to help us unwind whether this was … an active conspiracy Adam Schiff

“You’ve got Manafort trying to get money from this Russian oligarch … you have the Russians who want to have a relationship with the Trump campaign, they want to help Trump get elected. All those interests converge with Paul Manafort, so basically we want to know what can Manafort tell us about whether any of that was consummated.

“He’s trying to get money, they’re trying to get dirt, the Russians are trying to help Trump. Was there a meeting of the minds? That goes to the heart of the collusion or conspiracy issue.”

Schiff added: “Manafort is a key person to help us unwind whether this was the most improbable string of unlikely coincidences or whether this was an active conspiracy.”

Facebook Twitter Pinterest Adam Schiff is the senior Democrat on the House intelligence committee. Photograph: Michael Reynolds/EPA

The midterm elections are just over 50 days away and Republicans are struggling to hold on to a majority in the House, knowing that if Democrats take control it would open the way to impeachment proceedings.

Republicans who want to fight the election on tax cuts and a surging US economy have been unable to break through the noise around the mounting toll of convictions linked to the Mueller investigation.

Trump spent the weekend at the White House, mostly tweeting updates about the federal response after Hurricane Florence ravaged the Carolinas.

Barack Obama is back to fire up the base – Democrat and Republican Read more

But he returned to his own defence, tweeting on Sunday morning: “The illegal Mueller Witch Hunt continues in search of a crime. There was never Collusion with Russia, except by the Clinton campaign, so the 17 Angry Democrats are looking at anything they can find. Very unfair and BAD for the country. ALSO, not allowed under the LAW!”

Ken Starr, the special prosecutor whose investigation of the Monica Lewinsky affair 20 years ago led to the unsuccessful impeachment of Bill Clinton, said on CNN’s State of the Union: “The Trump White House and the lawyers are taking a page from the Clinton playbook. Attack the prosecutor.”

He said the real significance of Manafort’s move was “we are much closer to getting the truth than we were before this plea”, calling it “terrific for the investigation and frankly the American people”.

Starr, who has just published a book about the investigation of Clinton, said Trump would be unwise to give Manafort a pardon. Asked if impeachment should happen, he said: “I hope not, because one of the lessons in the book is impeachment is hell. The country should not be taken through that.

“The founding generation wisely knew that it was such a serious act that it would require a two-thirds majority in the Senate. Unless there is a growing national consensus that impeachment is proper, it’s doomed to fail and it’s just the wrong way to go.”

A CNN poll last week showed eight in 10 Democratic voters think Trump should be impeached immediately and, across the board, more voters approve of Mueller than Trump when it comes to handling of the Russia investigation.

Play Video 3:27 How to impeach a US president – video

Ken Starr: 'There are eerie echoes of the past' Read more

Mueller has been working for 16 months investigating Russia’s covert intervention in the 2016 campaign, links to members of Trump’s team and potential obstruction of justice by the president.

The Alabama Democratic senator Doug Jones told CNN: “Clearly you have people close to the president of the United States who have committed crimes and that, in and of itself, is a problem. But it’s not necessarily an impeachable offence.”

Jones, fighting to hold a seat in deep Trump country, cautioned that any judgment on whether to proceed with impeachment must wait until Mueller completes his work.

“Just because we’ve seen people that surround the president have gone forward [to prosecution] doesn’t mean there should be impeachment hearings, not by any stretch,” he said. “Once we see the reports we’ll have to weigh those reports on their own.”