The U.S. House of Representatives opened impeachment hearings Wednesday into a president's actions for just the fourth time in history, as the public began to hear about allegations that Donald Trump and his backers mounted an improper pressure campaign on the Ukraine government.

The first day of hearings provided hours of partisan back-and-forth but no singular moment to be etched in the public consciousness as grounds for removing the 45th president from office.

William Taylor, the top U.S. diplomat in Ukraine, testified that Trump was overheard asking about "the investigations" he wanted Ukraine to pursue, an issue central to the impeachment inquiry.

Taylor said a member of his staff recently told him he overheard the U.S. president when he was meeting with the U.S. ambassador to the European Union, Gordon Sondland, at a Kyiv restaurant, the day after Trump's July 25 phone call with the new leader of Ukraine, Volodymyr Zelensky.

The aide explained that Sondland had called the president from the restaurant and he could hear Trump on the phone asking about "the investigations."

Taylor said he took that to mean investigations into former vice-president and current Democratic presidential candidate Joe Biden and his son Hunter, who had served as a director on the board of Burisma Group, a Ukrainian energy company.

The ambassador told the president the Ukrainians were ready to move forward, Taylor testified.

When the aide later asked Sondland what Trump thought of Ukraine, "Ambassador Sondland responded that President Trump cares more about the investigations of [Joe] Biden, which [Rudy] Giuliani was pressing for," Taylor told the committee.

Taylor also said he was told Trump wanted the Ukrainian leader "in a public box," which, Taylor said, he understood to mean making a public statement about the investigations.

Trump told reporters last week he "hardly" knows Sondland.

At a White House news conference with Turkish President Tayyip Erdogan after Wednesday's hearing ended, Trump said he knew "nothing" about the call with Sondland that Taylor said his aide overheard.

"It's the first time I heard it," Trump said.

Watch Trump's exchange with the reporter:

A reporter asks U.S. President Donald Trump about the phone call diplomat William Taylor recounted for the first time during Wednesday's impeachment hearings. 1:01

He also told reporters he was too busy today to watch the hearings.

"I hear it's a joke. I haven't watched for one minute," Trump said. "This is a sham, and it shouldn't be allowed."

Adam Schiff, chair of the intelligence committee, said in his opening statement the hearings would 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."

U.S. chargé d'affaires for Ukraine Bill Taylor focuses on phone call in which President Donald Trump is advised about investigations he wanted Ukraine to pursue 1:02

Schiff, a California Democrat, alleged there was a concerted campaign operating outside the normal parameters of diplomacy to pressure Ukraine into announcing a pair of investigations the Trump administration desired.

Democrats 'turned on a dime': Nunes

Devin Nunes, the top Republican on the committee, dismissed the foundations of the investigation, characterizing it as a continuation of a partisan campaign to remove Trump from office that began in 2017 with a probe into allegations of conspiracy between the Trump campaign and transition teams and Russia.

"After the spectacular implosion of their Russia hoax on July 24, in which they spent years denouncing any Republican who ever shook hands with a Russian, on July 25 they turned on a dime and now claim the real malfeasance is Republicans' dealings with Ukraine," said Nunes, of California.

Top Republican on panel, Devin Nunes, blasts 'spectacle' of impeachment process 'in search of a crime' 0:59

He described the closed-door testimony that has been ongoing for several weeks since September as taking place in a "cult-like atmosphere."

It has been alleged that Trump and his allies, led by his personal lawyer Rudy Giuliani, pressured Zelensky — including in the July 25 phone call that triggered a whistleblower complaint — to investigate Biden and his son, who served on the board of Ukraine energy company Burisma, for corruption.

As well, Trump requested that Zelensky's officials investigate if Ukraine entities allied with the Hillary Clinton campaign during the 2016 presidential election were involved in the hacking of Democratic National Committee emails. A Republican-led Senate committee and U.S. intelligence agencies, among others, ascribe that breach to Russian actors.

The overriding question is whether Trump withheld $391 million US in military and security aid to Ukraine for nearly two months as he waited for Zelensky to commit to doing his bidding.

Schiff emphasized the vulnerability of Ukraine, which has seen thousands die in the past five years as it has dealt with a Russian-backed military campaign within its borders.

Taylor said while it was "hard to draw any direct lines" to deaths on the battlefield, the financial aid was crucial for the country's military.

Chair of U.S. House intelligence committee warns about ignoring alleged abuses of power 0:55

Responding to House intelligence committee counsel Daniel Goldman, Taylor said that in his years of experience he had never before seen aid tied to a president's personal interests.

Nunes said it was imperative the House hear from the whistleblower as well as Hunter Biden, requests the Democrats have denied.

Trump has insisted the call with Zelensky was "perfect." The White House released a rough summary around the same time the monetary aid was eventually released, in September.

I'm happy to have the person who started this thing come in and testify. President Trump is welcome to take a seat right there. - Peter Welch, Vermont Democrat

Republican Jim Jordan of Ohio seized on the fact Zelensky never did publicly announce the aforementioned investigations, nor was the financial aid denied.

While Republican counsel Steve Castor got Taylor to admit that Zelensky on July 26 described the phone call with Trump in positive terms, Schiff later got Taylor to admit Zelensky was loathe to be seen as meddling in U.S. domestic affairs.

George Kent, deputy assistant secretary of state for European affairs, testified there was no talk from the U.S. administration about helping Ukraine set up an anti-corruption unit or any other investigations beyond the Biden and DNC email probes.

In the day-long hearing that went from sombre to heated, Democrat Peter Welch of Vermont drew the biggest laughs.

After Jordan, alluding to the whistleblower, said Americans needed to hear from "the person who started this all," Welch retorted:

"I'd say to my colleague: I'd be glad to have the person who started it all come in and testify. President Trump is welcome to take a seat right there."

Witnesses dismiss partisan motives

Trump tweeted Wednesday that the witnesses were "NEVER TRUMPERS," but there has been no evidence suggesting they engaged in partisan activity.

Kent, in his opening statement, decried what he described as personal attacks on U.S. officials who have decided to testify.

U.S. State Department official George Kent testifies that Rudy Giuliani and 'corrupt' former prosecutors ran smear campaign against U.S. ambassador 1:05

Taylor emphasized his decades of work for both Republican and Democratic administrations.

"I am not here to take one side or the other or advocate for any particular outcome of these proceedings," he said

Taylor in previous testimony described an "irregular channel" of Ukraine outreach that included Giuliani, Sondland and Energy Secretary Rick Perry. More traditional diplomats on the file were occasionally kept out of the loop, Taylor has said.

"I think it's crazy to withhold security assistance for help with a political campaign," Taylor texted other officials on Sept. 9.

On Wednesday, Taylor reiterated that sentiment: "I believed that then, and I believe that now," he said.

For his part, Kent said he was alarmed by the Giuliani-led efforts to "gin up politically motivated investigations," specifically mentioning the recently indicted Guiliani associates Lev Parnas and Igor Fruman, as well as two former Ukraine prosecutors who Kent characterized as corrupt and involved in "peddling false information."

"As a general principle, I do not believe the United States should ask other countries to engage in selective, politically associated investigations or prosecutions," said Kent.

Both parties employed counsel for some questioning of the witnesses. Steve Castor, left, undertook questioning on behalf of House Republicans, while Daniel Goldman did so for the Democrats. (Saul Loeb/Reuters)

Democrats have been stymied in their request to hear from some witnesses, including Giuliani and Mick Mulvaney, the White House chief of staff and top budget official.

"If the president can simply refuse all oversight, particularly in the context of an impeachment proceeding, the balance of power between our two branches of government will be irrevocably altered," said Schiff.

"That is not what the [Constitution] founders intended."

2 previous impeachments

The U.S. Constitution stipulates the president and other officers of government "shall be removed from office on impeachment for, and conviction of, treason, bribery, or other high crimes and misdemeanours."

The House could eventually debate and vote on whether to bring forward charges, known as articles of impeachment. A simple majority of the House's 435 members would lead to an impeachment resolution.

WATCH: How Trump impeachment could play out "

These are the steps that could lead to U.S. President Donald Trump's removal from office. 3:27

A trial in the Senate, where Republicans have the majority, could then ensue.

Only two U.S. presidents have been impeached by the House: Andrew Johnson and Bill Clinton. Neither was convicted in a subsequent Senate trial.

Articles of impeachment were drafted in 1974, but after recordings implicated him in the Watergate break-in, Richard Nixon resigned before a House vote took place.

Nine more witnesses so far are scheduled to appear at the hearings through next week. On Friday, the committee will hear from Marie Yovanovitch, the U.S. ambassador to Ukraine ousted in the spring.

On Wednesday, Democrats got Kent and Taylor to agree there was a smear campaign directed at Yovanovitch of a kind they hadn't seen before in their experience.

One potential complication to the momentum the Democrats hope to build is that the U.S. government needs a funding bill to avoid a shutdown after Nov. 21.

WATCH | Missed the hearing? Watch the highlight reel: