But Trump's ambassador to the European Union, Gordon Sondland, and Trump's former special to Ukraine, Kurt Volker, insist they didn't realize that pushing Ukraine to investigate the company where Hunter Biden was a board member was code for investigating both Bidens.
It's an important distinction as Republicans defend Trump against the charge that he abused the power of his office for an investigation that would help him politically.
Testifying to the House impeachment inquiry, Sondland and Volker gave damaging accounts about on the central events on Ukraine. They said they believed a White House visit for Ukraine's new president, Volodymyr Zelenskiy, became conditioned on Ukraine making a public statement about investigations into the energy company Burisma and Democratic actions in the 2016 U.S. election. They were adamant that the link to Biden wasn't obvious.
Then in September, the White House released a rough transcript of Trump's July 25 call with Zelenskiy. Only now, according to Sondland and Volker, did it become clear to them.
"I was not made aware of any reference to Vice President Biden or his son by President Trump until the call was released on Sept. 25," Volker said. He said he found Biden to be an honorable man.
Other witnesses who testified this week at impeachment hearings by the House Intelligence Committee said they didn't necessarily make the link to Burisma, either. One former National Security Council aide, Tim Morrison, had to Google it.
"Apparently, a lot of people did not make the connection," Sondland testified Wednesday.
The committee chairman, Rep. Adam Schiff, D-Calif., questioned Sondland on Wednesday about whether he ever put two and two together to realize that looking into Burisma meant the Bidens, given all the things Giuliani was saying publicly.
"I didn't," Sondland said.
"I wasn't paying attention to what Mr. Giuliani was saying on TV," he said. "We were talking to him directly."
Democrats say there may be grounds for impeachment in Trump's push for Ukraine's new leadership to investigate Biden and the 2016 U.S. election as Trump withheld military assistance approved by Congress.
Trump on Wednesday insisted he wanted "nothing" from Ukraine and said the impeachment hearings should be brought to an end.
Republicans on the committee have tried to shift the spotlight away from Trump and to a debunked theory that Ukraine may have meddled in the 2016 elections and Hunter Biden's role overseas. The Bidens have not been accused of wrongdoing.
Hunter Biden was named a paid board member of Burisma Holdings in April 2014. The company's founder was a political ally of Viktor Yanukovych, Ukraine's Russia-friendly president, who was driven out in February 2014 by mass protests.
Yanukovych's ouster prompted the Obama administration to move quickly to deepen ties with Ukraine's new government. Joe Biden played a leading role, traveling to Ukraine and speaking frequently with its new Western-friendly president.
The younger Biden's business role raised concerns among anticorruption advocates that Burisma was seeking to gain influence with the Obama administration.
Hunter Biden has denied using his influence with his father to aid Burisma. He remained on the board through early 2019.
Trump and his allies point in particular to Joe Biden's move in March 2016 to pressure the Ukrainian government to fire its top prosecutor, Viktor Shokin, who had previously led an investigation into Burisma's owner.
But Biden was representing the official position of the U.S. government, a position that was also supported by other Western governments and many in Ukraine, who accused Shokin of being soft on corruption.
"I rejected the conspiracy theory that Vice President Biden would have been influenced in his duties as Vice President by money paid to his son," Volker testified. "I have known Vice President Biden for more than 24 years. He is an honorable man and I hold him in the highest regard."
Corruption has continued to fester in Ukraine. Zelenskiy came into office in May with no political experience but with bold promises to put an end to the corrupt practices.
Around this time, Giuliani began reaching out to Zelenskiy and his aides to press for a government investigation into Burisma and Hunter Biden's role with the company. His concerns became more vocal over time.
"The Washington press will not accept the fact that Joe Biden may have done something like this," Giuliani said on an interview on ABC.
During his testimony, Volker said he saw Burisma and Biden as separate.
"I did not understand that others believed that any investigation of the Ukrainian company, Burisma, which had a history of accusations of corruption, was tantamount to investigating Vice President Biden. I drew a distinction between the two."
"In retrospect I should have seen that connection differently, and had I done so, I would have raised my own objections," Volker testified.
Associated Press writers Mary Clare Jalonick and Lisa Mascaro contributed to this report.
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