IF THE CASE for impeaching President Donald Trump needed any further nailing down, it received it on October 29th. Lieutenant-Colonel Alexander Vindman, the senior expert on Ukraine at the National Security Council (NSC), told impeachment investigators that he was so appalled by Mr Trump’s repeated demands that Volodymyr Zelensky, the president of Ukraine, investigate Joe Biden, a front-runner for the Democratic presidential nomination, that he reported it to a lawyer at the NSC. He did so twice.

“I did not think it was proper to demand that a foreign government investigate a US citizen, and I was worried about the implications for the US government’s support of Ukraine,” Colonel Vindman said in his opening statement, which was published beforehand. He said he believed that if Ukraine did pursue an investigation into the Bidens and Burisma, a Ukrainian energy company on whose board Mr Biden’s son served while his father was vice-president, “it would likely be interpreted as a partisan play which would undoubtedly result in Ukraine losing the bipartisan support it has thus far maintained.”

Colonel Vindman’s testimony was given privately to three House committees—on Intelligence, Foreign Affairs, and Oversight and Reform—in defiance of a White House order not to co-operate with the impeachment inquiry launched by the Democrats last month. It is important for three reasons.

First, Colonel Vindman is the first official from the White House to testify that he listened in on the conversation between Presidents Trump and Zelensky on July 25th. Colonel Vindman has become at least the fourth person to suggest Mr Trump offered Mr Zelensky a quid pro quo. Last month, an account from a whistle-blower of that conversation prompted the Democrats to launch the inquiry. There followed testimony by William Taylor, the acting ambassador to Ukraine; Gordon Sondland, the ambassador to the European Union; and an ill-advised account by Mick Mulvaney, the White House chief of staff, which he then tried to retract. Colonel Vindman overheard the conversation, and reportedly told investigators that a rough transcript published in September, already implicating the president in seeking a favour, omitted passages that look even more damning.

Second, the colonel has given the strongest evidence so far that there was a well-established plan to discredit Mr Biden by getting him investigated by Ukraine. Colonel Vindman told investigators that during a meeting on July 10th with Ukrainian officials, Mr Sondland “started to speak about Ukraine delivering specific investigations in order to secure the meeting with the president.” At that point, Colonel Vindman recalled, John Bolton, the national security adviser at the time, “cut the meeting short.”

A little later when Mr Sondland “emphasised the importance that Ukraine deliver the investigations into the 2016 election, the Bidens and Burisma,” Colonel Vindman told Mr Sondland that “his statements were inappropriate, that the request to investigate Biden and his son had nothing to do with national security, and that such investigations were not something the NSC was going to get involved in or push.”

This corroborates testimony from Mr Trump’s former Russia adviser, Fiona Hill, to whom Colonel Vindman reported. Earlier this month she testified that she had been alarmed on July 10th when Mr Sondland referred to a probe of Mr Biden. Mr Taylor, the acting ambassador, also testified that Mr Sondland had privately noted that Mr Trump was demanding that Ukraine publicly announce an investigation into the Bidens.

Colonel Vindman’s testimony does not look good for Mr Sondland, whom it appears to contradict: earlier this month, Mr Sondland told investigators that he did not remember discussing the Bidens and that no one had raised concerns about Mr Trump’s actions.

Beyond this, it may be Colonel Vindman’s particular strength as a witness that makes his testimony most significant—and most damaging to the president. Colonel Vindman, who was born in what is now Ukraine, is a decorated veteran. He received a Purple Heart, a military medal, after being injured in Iraq by an improvised explosive device. In his opening statement, he described himself as a patriot for whom it is a “sacred duty and honour to advance and defend our country irrespective of party or politics”.

If Colonel Vindman testifies publicly, Americans may be less inclined to dismiss him, as Mr Trump would like, as a partisan witness. On October 31st the House of Representatives will vote on a resolution to “affirm” the impeachment inquiry, which will also outline procedures for holding public hearings. Mr Trump’s supporters seem to realise the damage Colonel Vindman could do. Commentators on Fox News suggested he was a Ukrainian spy. Mr Trump, meanwhile, attacked Colonel Vindman as a “Never Trumper”.

But in a possible sign of a crack in Mr Trump’s red wall, Liz Cheney, the number three Republican in the House, said that attacks on Colonel Vindman should stop. She blasted those, “questioning the patriotism, questioning the dedication to country of people like Mr Vindman…I think that we need to show that we are better than that as a nation.”

Clarification (October 30th 2019): Colonel Vindman’s reported comments on the transcript have been updated since the original publication of this piece