Donald Trump personally kept up pressure on Ukraine to carry out investigations he had requested in a phone call with a senior US diplomat, who then observed the president did not “give a shit” about Ukraine and only cared about what would benefit him politically, according to dramatic new testimony in congressional impeachment hearings.

In a deposition to the House committees investigating the Ukraine scandal, David Holmes, a diplomat at the US embassy in Kiev, described an extraordinary phone call between Trump and the US ambassador to the European Union, Gordon Sondland, on 26 July.

It was the day after Trump had spoken to the new Ukrainian president, Volodymyr Zelenskiy, requesting a “favour” and suggesting the Ukrainians investigate former vice-president Joe Biden, and his son, Hunter.

Sondland placed the 26 July call to Trump from a Kyiv restaurant, where he was having lunch with Holmes and two other US colleagues. According to Holmes, Trump was speaking so loudly that Sondland “held the phone away from his ear for a period of time”.

Sondland, a wealthy hotelier who donated $1m to Trump’s inauguration celebration, assured Trump that Zelenskiy “loves your ass”.

“I then heard President Trump ask, ‘So, he’s gonna do the investigation?’ Ambassador Sondland replied that ‘he’s gonna do it’,” adding that Zelenskiy will do “anything you ask him to”, Holmes testified in his opening statement to House investigators, which was leaked to US media.

After the call, Holmes asked Sondland about the president’s feelings about Ukraine, asking if it was true that Trump “did not give a shit” about Ukraine.

Holmes recalled: “Ambassador Sondland agreed that the President did not ‘give a shit about Ukraine’. I asked why not, and Ambassador Sondland stated that the president only cares about ‘big stuff’.”

Holmes went on: “I noted that there was ‘big stuff’ going on in Ukraine, like a war with Russia, and Ambassador Sondland replied that he meant ‘big stuff’ that benefits the president, like the ‘Biden investigation’ that Mr Giuliani was pushing.”

Rudy Giuliani, former New York mayor and Trump’s personal lawyer, had for some months been running a parallel negotiating channel with the Ukrainian government, which was focused on persuading Kyiv to investigate an energy company called Burisma, which had put Hunter Biden on its board, and announce another investigation into claims that Ukraine’s meddling on the Democrats’ behalf in the 2016 election. Those claims were based on conspiracy theories debunked by the US intelligence community.

The Trump-Sondland phone call was first revealed on Wednesday by Bill Taylor, the acting ambassador to Kyiv, who told investigators Holmes had informed him about it. The significance of the Holmes testimony is that it proves Trump was personally directing the pressure being exerted on Zelenskiy, and that Sondland was reporting to him on a daily basis. After Sondland amended his own testimony to admit that military aid to Ukraine was being made conditional on the specific investigations, Trump had sought to distance himself, claiming “I hardly know the gentleman”.

The 26 July call, to which there are reportedly other witnesses, also deepens Sondland’s legal jeopardy. He has already altered his testimony once, but had not disclosed this conversation with the president. He told investigators he had had a phone conversation with the president the previous day, but described it as “a nothing call”. Holmes also testified that he had been asked by the embassy to take notes at a meeting earlier on 26 July in Kyiv between Sondland and a Ukrainian presidential aide, Andriy Yermak, but by the time Holmes reached Yermak’s office he had been shut out, and told that Sondland and Yermak, had insisted on a one-to-one meeting with no note-taker present. Sondland also failed to disclose the contents of this meeting in his congressional testimony.

There will also be questions about his decision to place a call from his mobile phone to the president from a Kyiv restaurant. Such conversations are normally carried out from secure facilities on encrypted lines. Another impeachment witness, former national security council senior director Fiona Hill, has already testified on her concerns that Sondland was a counter-intelligence risk.

Holmes’s dramatic testimony capped a devastating week for Trump in terms of the evidence presented to the impeachment inquiry. Taylor provided detailed testimony about the efforts of Giuliani and others to compel Zelenskiy to dig up dirt on the Bidens. And on Friday morning, Taylor’s predecessor in Kyiv, Marie Yovanovitch, gave an account of how Trump and Giuliani had hounded her from her post.

Yovanovitch said she felt “shocked and devastated” by Trump’s personal attacks on her, and that she was “amazed” corrupt elements in Ukraine who saw her as an obstacle had found willing US allies, including the president. Trump himself illustrated her concerns by tweeting an attack on her during her testimony, bizarrely blaming the bloodshed and chaos in Somalia on Yovanovitch because she had been posted there as a junior diplomat some three decades ago.

However, Republicans closed ranks to defend Trump and attack his accusers, reinforcing expectations that the process is likely to be decided by party loyalty, with the Democrat-run House of Representatives voting to impeach the president, but the Republican-majority Senate, acting as jury, voting to acquit him.

On Saturday morning, House impeachment investigators met in private with a White House budget official, Mark Sandy, the first official from the Office of Management and Budget to defy Trump’s instructions not to testify.

Like others, he received a subpoena to appear.