George Kent, the State Department’s foremost expert on Ukraine, seemed destined for a supporting role in Wednesday’s impeachment hearings — more memorable for his matching blue-and-yellow bow tie and pocket square than any newsworthy moments.

But after initial rounds of questions from Republican and Democratic counsels, which were mostly directed at William B. Taylor Jr., the senior U.S. diplomat in Ukraine, Kent emerged as a forceful debunker of some of the most frequently cited assertions and conspiracy theories among President Trump’s allies.

Under questioning from Democrats, Kent said there was “no factual basis” of allegations that Ukraine intervened in the 2016 election, something that Trump asked Ukrainian President Volodymyr Zelensky to investigate when he brought up the computer security company CrowdStrike in the leaders’ July 25 phone call.

“I think it’s amply clear that Russian interference was at the heart of the interference in the 2016 election cycle,” Kent said, echoing the assessment of every U.S. intelligence agency, and every Trump-appointed head of those agencies.

Kent’s rejection of the notion that Ukraine intervened in the U.S. election to oppose Trump appeared to frustrate the top lawyer for the Republicans, Stephen R. Castor, who eventually stopped asking Kent about it.

“Ambassador Kent, you didn’t seem to be too concerned about it in the last round of questioning, so I’ll just skip you,” Castor said.

Kent also dispelled some lesser-known theories and talking points taken up by Trump’s defenders.

For instance, Democrats asked him to comment on the claim by Sen. Rand Paul (R-Ky.) that Trump’s threat to withhold aid to Ukraine was “exactly” the same tactic used by then-Vice President Biden when he threatened to withhold aid if Ukrainian prosecutor general Viktor Shokin wasn’t fired.

“I do not think they are the same thing,” Kent said. What Biden requested, Kent said, was the removal of “a corrupt prosecutor general . . . who had undermined a system of criminal investigation that we built with American money to build corruption cases.” Shokin, Kent said, had “destroyed the entire ecosystem that we were trying to create,” and he credited Biden for leading a U.S. effort to combat corruption in Ukraine.

Democrats also questioned Kent about the campaign waged by Trump’s personal lawyer, Rudolph W. Giuliani, to oust the former U.S. ambassador to Ukraine, Marie Yovanovitch. Giuliani and Ukraine’s ex-chief prosecutor, Yuri Lutsenko, have accused Yovanovitch of providing a “do not prosecute list” to Ukrainian officials to protect Biden and others.

Kent adamantly rejected those allegations, saying, “I have every reason to believe it is not true.” He described Lutsenko as a “corrupt” prosecutor with questionable motives and defended Yovanovitch as “dedicated, as is every U.S. government official in Ukraine, to help Ukrainians overcome the legacy of corruption.”

Kent did help Republicans establish that there was concern about the appearance of a conflict of interest when Biden’s son Hunter was given a seat on the board of the Ukrainian energy company Burisma while Biden led Ukraine policy for the Obama administration.

Kent said that he raised his concerns with Biden’s national security staff in a briefing call in February 2015 that the younger Biden’s position on the board “could create the perception of a conflict of interest.” Kent said he did not receive a response.

And Kent testified, “I did not witness any efforts by any U.S. official to shield Burisma from scrutiny. In fact, I and other U.S. officials consistently advocated reinstituting a scuttled investigation of [Mykola] Zlochevsky, Burisma’s founder, as well as holding the corrupt prosecutors who closed the case to account.”

In his closed-door deposition last month, Kent also expressed concerns about rampant corruption in Ukraine’s government and business community. Before serving at the embassy in Kyiv, Kent was the senior anti-corruption coordinator in the State Department’s European Bureau.

“If you took the roster of the richest Ukrainians, they didn’t build value, they largely stole it,” Kent told lawmakers in October. “Most of the billionaires in the country became billionaires because they acquired state assets for largely undervalued prices and engaged in predatory competition.”

Kent, a Harvard-educated diplomat whose wife is of Crimean-Tatar ancestry, is known for having a passion for Ukraine and a scholarly interest in world affairs. He played the role of history professor. In his opening statement, he compared U.S. military support of Ukraine to foreign support of another fledgling, revolutionary republic.

“Our 18th-century independence,” he said, referring to the post-colonial United States, “would not have been secured without the choice of European officers — the French-born Lafayette and Rochambeau, the German-born von Steuben, and the Poles Pulaski and Kosciuszko — to come to the New World and fight for our cause of freedom, and the birth of a new country free from imperial dominion.”