Donald Trump's acting chief of staff, Mick Mulvaney, has appeared to confirm that the president withheld military aid to Ukraine at least partly in order to press for an investigation into his political opponents.
The president is facing impeachment - and possible removal from office - for allegedly abusing his position by pressuring a foreign leader to dig up or even manufacture dirt on Joe Biden, one of the frontrunners to be the Democratic candidate in next year's election.
The White House has angrily denied withholding military aid to Ukraine in return for political dirt in any "quid pro quo".
But responding to questions from reporters at the White House on Thursday, Mr Mulvaney said of the president: "Did he also mention to me in the past that the corruption that related to the DNC server? Absolutely, no question about that. But that's it and that's why we held up the money."
Asked whether that constituted the kind of quid pro quo the White House has denied, Mr Mulvaney said: "We do that all the time with foreign policy."
He added: "Get over it. There's going to be political influence in foreign policy."
According to a partial transcript of the 25 July phone call with Mr Zelensky, Mr Trump discusses military aid, which had already been held up, before saying: "I would like you to do us a favour though because our country has been through a lot and Ukraine knows a lot about it.
"I would like you to find out what happened with this whole situation with Ukraine, they say Crowdstrike ... I guess you have one of your wealthy people ... The server, they say Ukraine has it. There are a lot of things that went on, the whole situation."
Crowdstrike is the California-based company that investigated the hacking of the Democratic servers that occurred in the lead-up to the 2016 election. The hack, which proved embarrassing to Hillary Clinton, has been blamed on Russia by US intelligence services, although Mr Trump has repeatedly questioned their assessment.
Instead he is said to believe that Ukraine was somehow involved and that the country has one of the servers - although there is so far no evidence for either claim.
His personal attorney, Rudy Giuliani, has been trying to investigate the whereabouts and background of the server, as well as alleged corruption by Joe Biden's son Hunter, who was on the board of a Ukrainian company. Both Bidens deny any wrongdoing and Mr Trump has so far failed to produce any evidence to back up his claims.
On Wednesday Mr Trump said he even believed his predecessor, Barack Obama, may have been involved in unspecified corruption in the lead-up to the 2016 election.
Listing some of his favourite targets, he said: "Well, I don't know the details; I just know that our country is looking into the corruption of the 2016 election. It was a corrupt election, whether it's Comey; or McCabe; or Strzok or his lover, Lisa Page - the two great lovers. There was a lot of corruption. Maybe it goes right up to President Obama. I happen to think it does."
He added: "Rudy was one of many people that was incensed at the corruption that took place during that election. Pure corruption.
"For instance, I still ask the FBI: Where is the server? How come the FBI never got the server from the DNC? Where is the server? I want to see the server. Let's see what's on the server. So, the server, they say, is held by a company whose primary ownership individual is from Ukraine. I'd like to see the server. I think it's very important for this country to see the server. Nobody wants to see it. The media never wants to see it. But I'll tell you, Republicans want to see it."
Crowdstrike supplied copies of the contents of the servers to the FBI and one of the servers has been put on public display in Washington.