Kurt Volker said he has known Biden as an honorable man for more than two decades, rebuffing debunked corruption allegations that Trump is said to have wanted the Ukrainians to investigate in exchange for military aid to hold off Russian aggression.

"The allegations against Vice President Biden are self-serving and non-credible," Volker declared.

Broader corruption in Ukraine was "plausible," but corruption by Biden wasn't, he said.

Volker testified alongside former White House national security official Tim Morrison in the second hearing of the day in the House's impeachment inquiry, only the fourth in history against a U.S. president.

Morrison, also requested by GOP members on the House Intelligence Committee, said at the outset that he was not there to question the "character or integrity" of any of his colleagues, though earlier Tuesday Republican lawmakers used his prior comments to try to discredit another witness, Lt. Col. Alexander Vindman. The White House even circulated a tweet that was an earlier quote by Morrison questioning Vindman's judgment.

Democrats say there may be grounds for impeachment in Trump's push for Ukraine's new leadership to investigate his Democratic rival and the 2016 U.S. election as he withheld military assistance approved by Congress.

Trump says he did nothing wrong and dismissed the hearings as a "kangaroo court."

Volker was the first person to come behind closed doors in the inquiry that started in September, resigning his position shortly before he did so.

Since then, a parade of witnesses has testified publicly and privately about what they recalled about the July 25 phone call between Trump and Ukraine's new leader, Volodymyr Zelenskiy. Many of those statements cast doubt on Volker's account that he didn't know the Ukraine gas company Burisma was tied to Biden, and that he wasn't aware of a possible quid pro quo offered by Trump.

A number of White House and State Department officials were listening to the call, but Volker was not.

On Tuesday, he said he opposed any hold on security assistance. And he said, "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."

Even though, he said, he understood that Biden's son Hunter had been a board member -- and he himself had been deeply involved with Ukrainian officials on a statement, never released, that would have committed the country to investigating Burisma and the 2016 U.S. election.

Volker himself requested a meeting on July 19 with Rudy Giuliani, Trump's personal lawyer, at which Giuliani mentioned accusations about the Bidens as well as the discredited theory that Ukraine interfered in the 2016 U.S. election.

He said he believes now, thanks to hindsight and the testimony of other witnesses, that Trump was using the aid to pressure Ukraine to investigate Hunter Biden and his role on the company's board.

"In retrospect I should have seen that connection differently, and had I done so, I would have raised my own objections," Volker testified.

Yet he also acknowledged that Trump never told him he was withholding aid for Ukraine unless there were investigations. He also said Ukrainians never told him that they wouldn't get a White House visit or military aid without committing to investigations. He said he would have objected had the president asked him to get Ukraine to do investigations.

Morrison, who stepped down from Trump's National Security Council shortly before he appeared behind closed doors last month, said he was not concerned that anything illegal was discussed on Trump's July 25 call with Ukraine's leader, testimony that Republicans have repeatedly highlighted.

"As I stated during my deposition, I feared at the time of the call on July 25th how its disclosure would play in Washington's political climate," he said Tuesday. "My fears have been realized."

He told lawmakers Tuesday that the transcript of the call was incorrectly placed in a highly secure location.

Democrats have seen ill intent in that action, but he said, "It was a mistake ... an administrative error."

Morrison has confirmed to investigators that he witnessed a key September conversation in Warsaw between Gordon Sondland, the U.S. ambassador to the European Union, and a Ukrainian official. Sondland told the official that U.S. aid might be freed if the country's top prosecutor "would go to the mike and announce that he was opening the Burisma investigation," Morrison said in previous closed-door testimony.

Volker shifted his account of a pivotal July 10 interaction at the White House. In his closed-door interview last month, he said there was no discussion of Giuliani's activities in Ukraine or investigations sought by the president.

But on Tuesday, he said the meeting was essentially over when Sondland made a "general" comment about investigations.

"I think all of us thought it was inappropriate; the conversation did not continue and the meeting concluded," Volker said.

A series of text messages Volker provided to lawmakers showed conversations between him, Sondland and another envoy in which they discussed a need for Ukraine to launch investigations, including into Burisma.

Volker said meeting with Giuliani was just part of the dialogue, and he had one in-person meeting with him, in which Giuliani "raised, and 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 also said a senior aide to Zelenskiy approached him last summer to ask to be connected to Giuliani. He said he made clear to the Zelenskiy aide, Andriy Yermak, that Giuliani was a private citizen and not a representative of the U.S. government.

He testified he wasn't part of an irregular foreign policy channel led Giuliani, as others have testified. He also rejected the idea that Trump dubbed him, Sondland and Energy Secretary Rick Perry the "three amigos" in charge of Ukraine policy.

"My role was not some irregular channel, but the official channel," Volker said.