In a deposition before the congressional impeachment committees Thursday, a national security council (NSC) official corroborated prior testimony describing efforts by the Trump administration to strike a suspicious deal with Ukrainians.

But the official, Tim Morrison, seemed to lay groundwork in his testimony for an exculpation of Donald Trump and suggested that Gordon Sondland, the US ambassador to the EU, was unilaterally conducting a rogue foreign policy in Ukraine.

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Deposed on Thursday in a closed-door session, Morrison, who is leaving his post as a top White House adviser on Russia and Europe, said that the ambassador Bill Taylor had accurately described an early September conversation between Sondland and Andriy Yermak, an assistant to Ukraine’s president, Volodymyr Zelenskiy.

“Ambassador Sondland told Mr Yermak that security assistance money would not come until President Zelenskiy committed to pursue the Burisma investigation,” Taylor testified, referring to $391m in US military aid for Ukraine suspended on Trump’s orders in mid-July, and to the Ukrainian gas company that once employed Hunter Biden, the son of the former vice-president and 2020 Democratic presidential candidate Joe Biden.

The “substance” of Taylor’s statement was “accurate”, Morrison testified – with a quibble.

“My recollection is that Ambassador Sondland’s proposal to Mr Yermak was that it could be sufficient if the new Ukrainian prosecutor general – not President Zelenskiy – would commit to pursue the Burisma investigation,” Morrison said, according to a copy of his opening statement first published by CBS News.

That basic demand by Sondland – that Ukraine announce investigations or face the loss of US military aid – is at the core of allegations that Trump used the power of his office to solicit foreign interference in the 2020 US election. The House of Representatives voted Thursday, almost precisely along party lines, to advance an impeachment inquiry based on those allegations.

Q&A How do you impeach the US president? Show Hide Article 1 of the United States constitution gives the House of Representatives the sole power to initiate impeachment and the Senate the sole power to try impeachments of the president. A president can be impeached if they are judged to have committed "treason, bribery, or other high crimes and misdemeanors" – although the US Constitution does not specify what “high crimes and misdemeanors” are.

The formal process starts with the House of Representatives passing articles of impeachment, the equivalent of congressional charges. According to arcane Senate rules, after the House notifies the Senate that impeachment managers have been selected, the secretary of the Senate, Julie Adams, tells the House that the Senate is ready to receive the articles. Then impeachment managers appear before the Senate to “exhibit” the articles, and the Senate confirms it will consider the case. The presiding officer of the Senate notifies the supreme court chief justice, John Roberts, of the impending trial. Roberts arrives in the Senate to administer an oath to members. The presiding officer will then administer this oath to senators: “I solemnly swear that in all things appertaining to the trial of the impeachment of Donald Trump, now pending, I will do impartial justice according to the constitution and laws, so help me God.” The Senate must vote on a resolution laying out ground rules for the trial including who the key players will be, how long they will get to present their cases and other matters. After the Senate is “organized”, the rules decree, “a writ of summons shall issue to the person impeached, reciting said articles, and notifying him to appear before the Senate upon a day and at a place to be fixed by the Senate”. A president has never appeared at his own impeachment trial. Trump will be represented by the White House counsel, Pat Cipollone, and his personal lawyer Jay Sekulow, among others. After the oath, the trial proper will begin. Senators may not speak during the proceedings but may submit written questions. The question of witnesses and other matters would be decided on the fly by majority vote. A time limit for the proceedings will be established in the initial Senate vote. The senators will then deliberate on the case. In the past this has happened behind closed doors and out of public view. The senators vote separately on the two articles of impeachment – the first charging Trump with abuse of power, the second charging him with obstruction of Congress. A two-thirds majority of present senators – 67 ayes if everyone votes – on either article would be enough to convict Trump and remove him from office. But that would require about 20 Republicans defections and is unlikely. The more likely outcome is a Trump acquittal, at which point the process is concluded. Two presidents have previously been impeached, Bill Clinton in 1998, and Andrew Johnson in 1868, though neither was removed from office as a result. Richard Nixon resigned in 1974 before there was a formal vote to impeach him. Tom McCarthy in New York

But in his testimony Thursday, Morrison, a former senior congressional aide, seemed to seek to exculpate Trump by suggesting the quid pro quo was all Sondland’s idea.

“I hoped that Ambassador Sondland’s strategy was exclusively his own and would not be considered by leaders in the administration and Congress, who understood the strategic importance of Ukraine to our national security,” Morrison said.

A lawyer for Sondland did not immediately respond to a request for comment. Lawyers for Sondland have said that he never mentioned the Bidens to the Ukrainians and suggested he was unaware of the Burisma-Biden tie.

Sondland is a hotelier with no diplomatic – or Washington bureaucratic – experience who was appointed ambassador after he gave $1m to Trump’s presidential campaign. Last week Sondland and his lawyer visited Capitol Hill to “review” a deposition Sondland gave on 17 October in which he said he took Trump at his word that there was no quid pro quo with Ukraine.

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In addition to raising concerns about Sondland, Morrison defended Trump. He repeated explanations Trump has previously given for the suspension of the military aid, saying that Trump was concerned about corruption in Ukraine and concerned that Europe was not doing enough.

Morrison also echoed the Trump argument that there could not have been a quid pro quo involving military aid with Ukraine because the Ukrainians were not aware the aid had been suspended at the time of a 25 July phone call in which Trump asked Zelenskiy for a political “favor”.

While it is unclear exactly when the Ukrainians realized the aid had been suspended, they knew by early August, according to a New York Times report. The issue surfaces in text messages among diplomats in late August.

Sondland’s dangling of that aid to extract the Biden investigation continued, meanwhile, into early September, when he joined the vice-president, Mike Pence, for bilateral meetings with Ukrainians in Warsaw, Poland.

“I have no reason to believe the Ukrainians had any knowledge of the review [of US military aid for Ukraine] until 28 August 2019,” the date Politico first reported on it, Morrison said.

Morrison additionally substantiated the accuracy of public summaries of the 25 July call, which he listened to at the time. Afterward, Morrison reported on the call to the top lawyer on the NSC, he testified.

Morrison said he approached the lawyer, who promptly moved the record of the call into a highly restricted archive, because Morrison was worried that the call would leak, resulting in partisan angst, confusion in Congress, and mistaken “Ukrainian perceptions of the US-Ukraine relationship”.

“I want to be clear,” Morrison testified, “I was not concerned that anything illegal was discussed.”