House of Representatives Speaker Nancy Pelosi said on Thursday that President Donald Trump already has admitted to bribery in the Ukraine scandal at the heart of a Democratic-led inquiry, accusing him of an impeachable offense under the United States Constitution.

"The bribe is to grant or withhold military assistance in return for a public statement of a fake investigation into the elections. That's bribery," Pelosi, the top Democrat in Congress, told a news conference the day after the first public hearing in the impeachment inquiry she announced in September.

"What the president has admitted to and says it's 'perfect,' I say it's perfectly wrong. It's bribery," Pelosi said.

Democrats are looking into whether the Republican president abused his power by withholding $391 million in US security aid to Ukraine as leverage to pressure Kiev to conduct two investigations that would benefit him politically. The money, approved by Congress to help a US ally combat Russia-backed separatists in the eastern part of the country, was later provided to Ukraine.

Trump has denied any wrongdoing.

Another central figure — former US Ambassador to Ukraine Marie Yovanovitch — is due to testify on Friday in the second public hearing in the inquiry.

White House budget official Mark Sandy will testify in the inquiry in a closed session on Saturday as scheduled if he is subpoenaed, his lawyer said on Thursday.

The inquiry threatens Trump's presidency even as he seeks re-election in November 2020. If the House approves articles of impeachment — formal charges — against Trump, the Senate would then hold a trial on whether to convict him and remove him from office. Republicans control the Senate and have shown little support for Trump's removal.

Pelosi's comments could offer a preview of articles of impeachment Democrats might put forward. At her news conference, she also said Trump's administration had committed "obstruction of Congress" by blocking testimony of officials summoned to testify in the inquiry.

The Constitution states that impeachable offenses include "treason, bribery or other high crimes and misdemeanors". Democrats have begun to use the words bribery or attempted bribery in discussing Trump's actions. According to precedent, obstruction could be another article of impeachment.

Republicans have said House Democrats already have decided to pass articles of impeachment, but Pelosi denied that was the case, saying the inquiry must play out before any decision can be made.

The focus of the inquiry is a July 25 phone call in which Trump asked Ukrainian President Volodymyr Zelenskiy to investigate Democratic rival Joe Biden and the former vice president's son Hunter, who had served as a board member for a Ukrainian energy company called Burisma. Trump also asked Zelenskiy to investigate a debunked conspiracy theory embraced by some Trump allies that Ukraine, not Russia, interfered in the 2016 US election.

Pelosi compared Trump's actions with former President Richard Nixon's conduct in the Watergate corruption scandal that led him in 1974 to become the only US president to resign. Pelosi said Trump's actions to enlist a foreign power to help him in a US election and the obstruction of information about that — she called it a cover-up — "makes what Nixon did look almost small".

Televised hearings

Republicans kept up their attacks on Adam Schiff, the Democratic chairman of the House Intelligence Committee, which is holding the public hearings. House Republican leader Kevin McCarthy accused Schiff of lying at Wednesday's hearing about not knowing the identity of the whistleblower within the US intelligence community whose complaint about Trump's call triggered the impeachment inquiry.

An estimated 13.8 million viewers across 10 broadcast and cable television networks watched the first day of proceedings, according to Nielsen ratings data.

Two career US diplomats, William Taylor and George Kent, testified on Wednesday. Taylor, the acting ambassador to Ukraine, offered an account that linked Trump more directly to the pressure campaign on Ukraine.

The Intelligence Committee is due to hear on Friday from Yovanovitch, whom Trump abruptly removed from her post as ambassador to Ukraine in May. Trump's personal lawyer, Rudy Giuliani, was working at the time to persuade Ukraine to carry out the two investigations.

Yovanovitch told lawmakers behind closed doors on October 11 that Trump ousted her based on "unfounded and false claims by people with clearly questionable motives" after she came under attack by Giuliani.

On Wednesday, Taylor offered a new disclosure that indicated Trump's keen interest in the investigations in Ukraine, saying a member of his staff overheard a July 26 phone call at a restaurant in which Trump asked about the probes the president had asked Zelenskiy to conduct a day earlier.

After the call between Trump and Gordon Sondland, a former political donor appointed as the US envoy to the European Union, the staff member asked Sondland what Trump thought about Ukraine, Taylor said.

"Ambassador Sondland responded that President Trump cares more about the investigations of Biden, which Giuliani was pressing for," Taylor testified.

Republican lawmakers called Taylor's account hearsay.

A second staffer for the US Embassy in Kiev overheard the phone call between Trump and Sondland, the Associated Press reported on Thursday, citing an unnamed source briefed on the situation.

Trump told reporters after the hearing that he knew "nothing" about the call with Sondland.

The staffer cited by Taylor is David Holmes, a Taylor aide who has been subpoenaed to testify in the inquiry on Friday behind closed doors, said a person familiar with the issue.

Democrats are aiming to settle the question of Trump's impeachment before the end of the year.