WASHINGTON (AP) — A top American diplomat revealed new evidence Wednesday of President Donald Trump’s efforts to press Ukraine to investigate political rivals as House investigators launched public impeachment hearings for just the fourth time in the nation’s history.

William Taylor, the highest-ranking U.S. official in Ukraine, said for the first time that Trump was overheard asking another ambassador about “the investigations” he’d urged Ukraine’s leader to conduct one day earlier. Taylor said he learned of Trump’s phone call with the ambassador only in recent days.

Republicans retorted that the Democrats still have no more than second- and third-hand knowledge of allegations that Trump held up millions of dollars in military aid for the Eastern European nation facing Russian aggression. Trump is accused of trying to trade that aid for Ukrainian investigations into former Vice President Joe Biden and the Democratic National Committee.

The hearing, the first on television for the nation to see, provided hours of partisan back-and-forth but so far no singular moment etched in the public consciousness as grounds for removing the 45th president from office. Trump, who was meeting at the White House with Turkish President Recep Tayyip Erdogan, declared he was “too busy” to watch.

The long day of testimony unfolded partly the way Democrats leading the inquiry wanted: in the somber tones of two career foreign service officers who described confusion both within the U.S. government and in Ukraine about what Trump wanted from Kyiv. Taylor testified alongside George Kent, a deputy assistant secretary at the State Department.

Taylor said his staff recently told him they overheard Trump’s phone call with another diplomat, Ambassador Gordon Sondland, at a restaurant the day after Trump’s July 25 phone call with the new leader of Ukraine that sparked the impeachment investigation. The staffer explained that Sondland had called the president and Trump could be heard asking about “the investigations.” Sondland told the president the Ukrainians were ready to move forward, Taylor testified.

The impeachment inquiry was launched after an anonymous whistleblower’s complaint about Trump’s dealings with Ukraine, including a July phone call in which he urged Ukrainian President Volodymyr Zelenskiy to investigate unfounded corruption allegations into Biden and Biden’s son -- all while the U.S. was holding up U.S. military aid.

At the start, Rep. Adam Schiff, the Democratic chairman of the Intelligence Committee, outlined the question at the core of the impeachment inquiry -- whether the president used his office for personal political gain.

“The matter is as simple and as terrible as that,” said Schiff of California. “Our answer to these questions will affect not only the future of this presidency but the future of the presidency itself, and what kind of conduct or misconduct the American people may come to expect from their commander in chief.”

Republicans lawmakers immediately pushed Democrats to hear in closed session from the anonymous whistleblower. Schiff denied the request at the time but said it would be considered later.

“We will do everything necessary to protect the whistleblower’s identity,” Schiff declared.

The top Republican on the panel, Rep. Devin Nunes of California, said Trump had a “perfectly good reason” for wanting to investigate the role of Democrats in 2016 election interference, giving airtime to a theory that runs counter to mainstream U.S. intelligence which found that Russia intervened and favored Trump.

Nunes accused the Democratic majority of conducting a “scorched earth” effort to take down the president after the special counsel’s Russia investigation into the 2016 election failed to spark impeachment proceedings.

“We’re supposed to take these people at face value when they trot out new allegations?” said Nunes, a top Trump ally.

Nunes called the Ukraine matter a “low rent” sequel to the Russia probe. “Democrats are advancing their impeachment sham,” he said.

Both Taylor and Kent defied White House instructions not to testify. They both received subpoenas to appear.

The veteran foreign service officers delivered heartfelt history lessons about Ukraine, a young and hopeful democracy, situated next to Russia but reaching out to the West.

Asked about a text message released earlier in the probe in which Taylor called it “crazy” to withhold the security aid to a foreign ally, he said, “It was illogical. It could not be explained. It was crazy."

Kent, in his opening remarks, directly contradicted a core complaint against Joe Biden being raised by allies of the White House, saying he never heard any U.S. official try to shield a Ukraine company from investigations.

Kent acknowledged that he himself raised concerns in 2015 about the then vice president’s son, Hunter Biden, being on the board of Burisma, a Ukraine gas company. He warned that it could give the “perception of a conflict of interest.” But Kent indicated no one from the U.S. was protecting the company from investigations in Ukraine as Republicans have implied.

“Let me be clear; however, I did not witness any efforts by any U.S. official to shield Burisma from scrutiny,” Kent said.

He did not go into detail about the issues central to the impeachment inquiry, but he voiced his concerns with them.

“I do not believe the United States should ask other countries to engage in selective, politically associated investigations or prosecutions against opponents of those in power, because such selective actions undermine the rule of law regardless of the country,” he said.

So far, the narrative being unspooled in weeks of investigations for the inquiry is splitting Americans, mostly along the same lines as Trump's unusual presidency. The Constitution sets a dramatic but vague bar for impeachment, and there's no consensus yet that Trump's actions at the heart of the inquiry meet the threshold of "high crimes and misdemeanors."

At its core, the inquiry stems from Trump’s July 25 phone call with Ukraine's newly elected president, Volodymyr Zelenskiy when he asked the Zelenskiy for “a favor.”

Trump wanted the Ukraine government to investigate Democrats in the 2016 election and his potential 2020 rival, Joe Biden.

The anonymous whistleblower first alerted officials to concerns about the phone call. The White House released a rough transcript of the conversation, with portions deleted.

Democratic House Speaker Nancy Pelosi was initially reluctant to launch a formal impeachment inquiry. But she pressed ahead in September after the whistleblower’s complaint.

Over the past month, witness after witness has appeared behind closed doors to tell the investigators what they know.

Whether Wednesday’s proceedings begin to end a presidency or help secure Trump's position, it was certain his chaotic term had finally arrived at a place he could not control and a force, the constitutional system of checks and balances, that he could not ignore.

Unlike the Watergate hearings and Richard Nixon, there is not yet a "cancer-on-the-presidency" moment galvanizing public opinion. Nor is there the national shrug, as happened when Bill Clinton's impeachment ultimately didn't result in his removal from office. It's perhaps most like the partisanship-infused impeachment of Andrew Johnson after the Civil War.

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Associated Press writers Colleen Long, Mike Balsamo, Eric Tucker, Laurie Kellman, Alan Fram, Zeke J. Miller and Matthew Daly in Washington contributed to this report.