Veteran U.S. diplomats William Taylor and George Kent testified before House impeachment investigators Wednesday, saying they witnessed a backchannel policy involving Ukraine that threatened the development of the former Soviet republic’s evolution as a strong democracy.

President Trump called the hearings the “single greatest scam in the history of American politics.”

The pair delivered lengthy opening statements as House Democrats opened the public phase of their investigation into whether President Trump illegally pressured Ukraine to do his political bidding in exchange for military aid approved by Congress to counter Russian aggression in Ukraine’s eastern provinces.

Taylor, the acting U.S. ambassador to Ukraine, described a phone conversation that took place a day after Trump’s July 25 phone call to Ukraine President Volodymyr Zelensky during which Trump asked Zelensky to do him a favor and investigate former Vice President Joe Biden, Biden’s son, Hunter, and a debunked conspiracy involving Ukraine intervention in the 2016 U.S. presidential election.

Taylor said a member of his staff overheard a phone call between Trump and U.S. Ambassador to the European Union Gordon Sondland during which Trump asked Sondland asked about the status of "the investigations." Taylor quoted Sondland as saying Trump "cares more" about the investigations than Ukraine.

Both Taylor and Kent said they understood $391 million military aid, which had been frozen, and an Oval Office visit by Zelensky were dependent on the investigations going forward with Zelensky’s backing.

Taylor called the administration’s approach to U.S. relations to Ukraine irregular and described the withholding of military as “crazy” and “illogical.” He said the security assistance was important because it was also in U.S. interests to show Russia it would not be allowed to bully its neighbor.

Kent testified he was familiar with Trump's corruption allegations against Biden and that they were unfounded. He said he raised concerns early on that Hunter Biden's appointment to the board of Ukraine energy company Burisma could be construed as a conflict of interest.

“I raised my concern that Hunter Biden’s status as board member could create the perception of a conflict of interest,” Kent testified, emphasizing he “did not witness any efforts by any U.S. official to shield Burisma from scrutiny.”

Trump said he's too busy to watch the hearings during an Oval Office meeting with Turkey's president.

“I’m too busy to watch it. It’s a witch hunt, it’s a hoax, I’m too busy to watch it. So, I’m sure I’ll get a report,” Trump told reporters.

Trump laid into House Speaker Nancy Pelosi shortly before the public phase of the House impeachment investigation opened, saying she “cares more about power than she does about principle.”

Trump maintains his phone call to Ukraine President Volodymyr Zelensky was “perfect.” During that July 25 call, Trump asked Zelensky to investigate former Vice President Joe Biden, Biden’s son and a debunked conspiracy theory that posits Ukraine was responsible for 2016 presidential election interference rather than Russia.

Trump’s twitter rant said Democrats had turned the impeachment process into a political “cudgel” and described witnesses as “never Trumpers,” and called the proceedings a “circus” and “phony showtrial.”

Republicans took to Twitter to criticize the proceedings and House Intelligence Committee Chairman Adam Schiff, alleging he was engaged in a coverup and was conducting a show trial.