After another tumultuous week in Washington, with the prospect of impeachment growing by the day, Donald Trump faced a stinging rebuke from the man who holds the president’s fate in his hands: the Senate majority leader, Mitch McConnell.

In a column for the Washington Post, the Kentucky Republican lambasted the president for making “a grave strategic mistake” in seeking to withdraw US troops from Syria, a move which allowed Turkey to attack Kurdish forces previously allied with the US against the Islamic State.

The impeachment inquiry in the House is focused on Trump’s attempts to have Ukraine investigate Joe Biden, a leading contender for the Democratic nomination. Testimony from key Trump aides has brought closer a vote on impeachment and thus a Senate trial.

A two-thirds majority would be needed to convict and remove the president. The New York Times reported this week that McConnell has begun preparing his caucus, offering a PowerPoint presentation “complete with quotes from the constitution, as he schooled fellow senators on the intricacies of a process he portrayed as all but inevitable”.

There is no substitute for American leadership Mitch McConnell

Most observers, including the New York Times, think McConnell’s grip on a party loath to break with Trump renders conviction unlikely. McConnell has vowed to stop it.

But as some Republican moderates will face pressure at the ballot box over impeachment, so foreign policy remains a key GOP interest, particularly the need for the US to look strong and to fight Islamic extremists.

This week, as the Trump administration scrambled to contain the damage in Syria, fighting continued and Russian troops moved in to fill the vacuum, dismay over Trump’s capitulation to Recep Tayyip Erdoğan spread among Republican senators.

McConnell’s rebuke was both oblique and partisan: he did not mention Trump by name and he did mention, and blame, Barack Obama three times. But it was nonetheless a rebuke.

“There is no substitute for American leadership,” he wrote. “No other nation can match our capability to spearhead multinational campaigns that can defeat terrorists and help stabilise the region. Libya and Syria both testify to the bloody results of the Obama administration’s ‘leading from behind’.”

McConnell also made pointed reference to bipartisan agreement in the Senate.

“In January,” he wrote, “following indications that the president was considering withdrawing US forces from Syria and Afghanistan … the Senate stepped up. A bipartisan supermajority of 70 senators supported an amendment I wrote [which] stated our opposition to prematurely exiting Syria or Afghanistan.”

McConnell said he had been “disheartened that nearly all the Senate Democrats running for president” did not back the amendment but “the consensus position of nearly all Republicans and a number of Democrats was encouraging”.

Democratic presidential contenders support the impeachment process. No Republican senators have yet said they do – the former Ohio governor and 2016 presidential hopeful John Kasich did so on Friday – but criticism of Trump, not only by his frequent critic Mitt Romney of Utah but from allies such as Lindsey Graham of South Carolina, is more often to be heard.

The former senator Jeff Flake has said that if a vote were held in private, as many as 35 Republicans would vote to remove Trump. Presuming all Democrats and independents did so, 20 defections would be enough.

Donald Trump has continued to lash out on Twitter over the impeachment inquiry. Photograph: Evan Vucci/AP

A rolling drumbeat of reports and revelations continues to increase pressure that has caused Trump, characteristically, to lash out on Twitter, backing his position on Syria, attempting to question the validity of the impeachment inquiry and attacking its leaders in viciously personal terms.

On Saturday morning, the president opened with simply: “#StopTheCoup.”

He had reason to tweet more. On Friday, CNN reported that Rudy Giuliani, the former New York mayor commonly described as Trump’s personal lawyer and a leading player in the president’s Ukraine policy, had pressed the White House and state department to grant a visa to Viktor Shokin.

Shokin, formerly Ukraine’s top prosecutor, was removed under international pressure led by Biden as vice-president to Obama, because he was seen as weak on corruption.

Trump and his allies allege that Biden wanted to relieve pressure on his own son, Hunter Biden, who was on the board of a Ukrainian energy company. There is no evidence of wrongdoing by either Biden but Hunter Biden has been moved to defend himself, relinquishing a position tied to China and saying this week he regretted his involvement in what he called a “swamp” in Ukraine.

Giuliani has said Shokin promised to reveal “dirt” on Democrats.

CNN’s report was based on testimony to House committees by George Kent, a state department official. Others who have offered testimony damaging to Trump include the former Ukraine ambassador Marie Yovanovitch; the British-born former White House adviser Fiona Hill; and the current European Union ambassador, Gordon Sondland.

A battle over access to records pertaining to Ukraine goes on. This week, Giuliani said he would not comply with a subpoena. The energy secretary, Rick Perry, embroiled in the Ukraine scandal, announced his resignation. But the most damaging blow came from inside the White House.

The key issue is whether Trump offered Ukrainian president Volodymyr Zelenskiy a quid pro quo: an unfreezing of nearly $400m in US military aid in return for what Trump called in a 25 July phone call “a favour”.

On Thursday, the acting White House chief of staff, Mick Mulvaney, suggested there had been such a deal, and told reporters to “get over it” – a slogan which swiftly appeared on T-shirts as the Trump fundraising juggernaut rolled on.

Amid reported dismay in Trump’s inner circle, Mulvaney tried to walk his comments back. He did not seem to succeed.

“It’s not an Etch-A-Sketch,” Francis Rooney, a Florida Republican, told the New York Times, miming use of the toy, which lets children erase their doodles and start again. “There were a lot of Republicans looking at that headline yesterday when it came up. I certainly was.”