(CNN) The top US diplomat in Ukraine told Congress Wednesday of a previously unknown conversation President Donald Trump had the day after his phone call with the Ukrainian President, in which Trump asked the US ambassador to the European Union about Ukraine opening investigations that would help him politically.

The testimony from diplomat Bill Taylor provides new details about the President's personal involvement in the push for Ukraine to open investigations into his political rivals as House Democrats began public impeachment hearings that could lead to the House voting to impeach a president for just the third time in US history.

Taylor testified that Gordon Sondland, the EU ambassador, told an aide that Trump's interest in Ukraine was the "investigations of Biden," and that he cared more about an investigation of former Vice President Joe Biden than he did about Ukraine.

Taylor testified that his aide told him of a phone conversation Trump had with Sondland on July 26, one day after Trump's phone call with Ukrainian President Volodymyr Zelensky . Taylor's aide, who was accompanying Sondland to meetings in Kiev with Ukrainian officials, could hear Trump asking Sondland about the investigations, Taylor testified. Sondland "told President Trump that the Ukrainians were ready to move forward," Taylor said.

Taylor testified that his aide later asked Sondland what Trump thought of Ukraine.

"Ambassador Sondland responded that President Trump cares more about the investigations of Biden, which (Trump personal attorney Rudy) Giuliani was pressing for," Taylor said.

House Intelligence Chairman Adam Schiff, a California Democrat, asked Taylor if Sondland's statement meant that Trump cared more about the investigations "than he does about Ukraine?"

"Yes, sir," Taylor responded.

The House took a historic step forward Wednesday in its impeachment inquiry of Trump with public hearings that Democrats hope will show the public that the President abused the power of his office to obtain dirt on a potential 2020 rival while withholding vital security aid from Ukraine. Wednesday's hearing is the first of five days of impeachment hearings House Democrats have scheduled before they decide whether to move forward with putting forward and potentially voting on articles of impeachment against the President.

Taylor and Deputy Assistant Secretary of State George Kent, who also testified Wednesday, both received subpoenas to testify, according to an official working on the impeachment inquiry, which also were issued before their closed-door depositions last month.

House impeachment investigators also announced Wednesday they had scheduled two new depositions later this week, including David Holmes, an aide to Taylor. Schiff confirmed after the hearing that Holmes was the unnamed aide Taylor was referring to who overheard Sondland's conversation with Trump.

He said that Taylor's new testimony showed that Trump was directing the push for investigations.

"There is an effort apparently to, by the President's allies, throw Sondland under the bus, throw (acting chief of staff Mick) Mulvaney under the bus, throw anybody under the bus in an effort to protect the President," Schiff said. "But what this call indicates, as other testimony has likewise indicated, is that the instructions are coming from the President on down."

Republicans argued that Taylor and Kent's testimony was all based on secondhand information, repeatedly pointing out they didn't speak to Trump, Mulvaney or Giuliani. And they argued that Ukraine ultimately received the US aid that had been frozen without opening an investigation.

"The transcript speaks for itself. There was no conditionality or quid pro quo in the transcript," said Rep. Jim Jordan, an Ohio Republican added to the Intelligence Committee for the impeachment hearings. "The two guys on the call have both been very clear, no pressure, no pushing, no linkage to investigations in both what President Zelensky and President Trump have said, and of course President Zelensky didn't pledge to do any investigations prior to the aid being released."

Diplomats testify Giuliani's efforts were 'infecting' US-Ukraine policy

Taylor is one of the most important witnesses for the Democrats' impeachment case. He first testified behind closed doors last month that he was told Trump held up US security aid and denied a one-on-one meeting with the Ukrainian President unless Ukraine announced an investigation into his political rivals. Taylor said he was told by officials who spoke with the President that Trump wanted Zelensky to "go to a microphone" to announce the investigation.

In Wednesday's testimony, Taylor explained that Giuliani's efforts led to an "irregular" policy channel with Sondland, former US special envoy Kurt Volker and acting chief of staff Mick Mulvaney, which he said was "running contrary to the goals of longstanding US policy."

Kent's testimony also expressed alarm at Giuliani's efforts — which he described last month as a " campaign of lies " — that led to the ouster of US Ambassador to Ukraine Marie Yovanovitch and then the push for investigations.

"In mid-August, it became clear to me that Giuliani's efforts to gin up politically-motivated investigations were now infecting US engagement with Ukraine, leveraging President Zelensky's desire for a White House meeting," Kent said.

Both Taylor and Kent emphasized their careers in public service. Kent defended Yovanovitch and other US diplomats who have come under attack, adding that Giuliani's shadow efforts "hampered US efforts to establish rapport with the new Zelensky administration."

Taylor began his statement by stating that he was not testifying at Wednesday's hearing "to take one side or the other, or to advocate for any particular outcome of these proceedings." Taylor bristled when Jordan stated that he was the Democrats' star witness, saying he was only there to tell Congress what he knew.

"My sole purpose is to provide facts as I know them about the incidents in question as well as my views about the strategic importance of Ukraine to the United States," Taylor said.

At the conclusion of the hearing, Schiff acknowledged there were "no shortage of strong feelings about what this means to the country" from both sides.

"At the end of the day we're going to have to decide based on the evidence that you and others provide whether we're prepared to accept in the presence of the United States, a situation where the president for their own personal or political benefit can condition military aid, diplomatic meetings or any other performance of an official act in order to get help in their reelection," Schiff said.

GOP pushes back on Taylor's direct knowledge

Republican lawmakers took aim at Taylor and Kent's lack of firsthand knowledge about the freezing of Ukraine aid. Several lawmakers pointed to the fact that neither diplomat had any conversation with the President himself or Mulvaney, and were basing their testimony on what others had told them.

"We got six people having four conversations in one sentence, and you just told me this is where you got your clear understanding? ... I mean, I've seen church prayer chains that are easier to understand than this," said Jordan. "With all due respect ambassador, your clear understanding was obviously wrong. Because it didn't happen."

Jordan and Rep. John Ratcliffe of Texas, who led much of the GOP questioning, took a more combative tone than Steve Castor , the GOP staff attorney who questioned the diplomats for the first block of the hearing. The Republican lawmakers pointed to Zelensky's comments that he felt no pressure and the fact that Ukraine ultimately received the aid in September.

"No pressure, no demands, no conditions, nothing corrupt. Nothing. Nothing on the call. That's what we heard President Zelensky say," Ratcliffe said.

Democrats pushed back by arguing that other officials who had firsthand knowledge, like Mulvaney and former national security adviser John Bolton, had been blocked from testifying by the White House.

In his opening statement, Rep. Devin Nunes, the ranking Republican on the Intelligence Committee, accused Democrats of undertaking a "scorched-earth war against President Trump," saying they were using a "carefully orchestrated media smear campaign" to impeach the President.

"One of the mothers of all conspiracy theories is that somehow the President of the United States would want a country that he doesn't even like, that he doesn't want to give foreign aid to, to have the Ukrainians start an investigation into Bidens," Nunes said.

Republicans focused many of their questions on Biden's son Hunter Biden serving on the board of Burisma, the Ukrainian energy company, as well as the notion pushed by Giuliani and his allies that it was Ukraine that interfered in the 2016 US election. There's no evidence of wrongdoing involving the Bidens and Ukraine, and the conspiracy theory that Ukraine, not Russia, interfered in the 2016 election is unsubstantiated.

Schiff, Republicans clash over whistleblower

Before Kent and Taylor began speaking, Republicans expressed their complaints to Schiff about the process of the hearing, pushing for Schiff to issue a subpoena to the anonymous whistleblower whose complaint sparked the Democratic impeachment inquiry. Democrats blocked a motion from Republicans along party lines after the hearing to subpoena the whistleblower for a deposition.

"Now, there is one witness that they won't bring in front of us. They won't bring in front of the American people. And that's the guy who started it all. The whistleblower," Jordan said. "Only Chairman Schiff knows who the whistleblower is. We don't. We will never get the chance -- we will never get the chance to see the whistleblower raise his right hand, swear to tell the truth and nothing but the truth -- we'll never get that chance. More importantly, the American people will never get that chance."

Republicans repeatedly pointed to the fact that a member of Schiff's staff was in contact with the whistleblower before the complaint was filed that sparked the Democratic impeachment inquiry, arguing that as a result Schiff knew the identity of the whistleblower. And there were procedural scuffles over questions surrounding the whistleblower.

While Schiff was misleading when he said his committee hadn't contacted the whistleblower during an interview in the fall, he has insisted he was not told the whistleblower's identity. His staffer told the whistleblower to hire an attorney and reach out to the inspector general.

"Some of my colleagues made the statement repeatedly that I've met with the whistleblower, that I know who the whistleblower is. It was false the first time they said it. It was false the second through 40th time they said it. It will be false the last time they say it," Schiff said.

The whistleblower's lawyers Andrew Bakaj and Mark Zaid later reacted in a series of tweets to Republican lawmakers' comments about their client.

"Watching today's hearing certain members repeatedly criticized and belittled the significance of hearsay information, yet those same members then pushed to subpoena the #Whistleblower who they assert only had second-hand info," Bakaj tweeted. "The purpose? To unmask the identity and intimidate."

Zaid echoed the sentiment, tweeting , "I watched today's hearing & repeatedly heard Members criticize & belittle significance of hearsay info. Yet same Members pushed to subpoena #whistleblower who they complain only has second hand info. #hypocrisy. Only reason is to intimidate & harm."

This story has been updated.