WASHINGTON -- House Democrats are refining part of their impeachment case against the president to a simple allegation: Bribery.

House Speaker Nancy Pelosi on Thursday brushed aside the Latin phrase "quid pro quo" that Democrats have been using to describe President Donald Trump's actions toward Ukraine. As the impeachment hearings go public, they're going for a more colloquial term that may resonate with more Americans.

"Quid pro quo: Bribery," Pelosi said about Trump's July 25 phone call in which he asked Ukrainian President Volodymyr Zelenskiy for a favour.

Trump says the call was perfect. Pelosi said: "It's perfectly wrong. It's bribery."

The House has opened its historic hearings to remove America's 45th president, with more to come Friday, launching a political battle for public opinion that will further test the nation in one of the most polarizing eras of modern times.

Democrats and Republicans are hardening their messages to voters, who are deeply entrenched in two camps.

Trump continued to assail the proceedings as "a hoax" on Thursday, and House GOP Leader Kevin McCarthy dismissed the witness testimony as hearsay, at best second-hand information.

The president, who said he was too busy to watch the initial hearing as it was televised, caught up in the White House residence Wednesday evening and tweeted along with a Fox News morning recap Thursday.

The president flatly denied the latest revelations. During Wednesday's hearing a diplomat testified that another State Department witness overheard Trump asking about Ukraine investigations the day after his phone call with Kyiv.

"First I've heard of it," he said, brushing off the question at the White House.

The Associated Press reported Thursday that a second U.S. Embassy official also overheard Trump's conversation.

While Trump applauded the aggression of some of his GOP defenders, he felt that many of the lawmakers could have done more to support him and he pressed that case with congressional allies ahead of the next hearing, according to Republicans who were not authorized to speak publicly about private conversations and were granted anonymity.

On Friday, Americans will hear from Marie Yovanovitch, the career foreign service officer whom Trump recalled as the U.S. ambassador to Ukraine after what one State Department official has called a "campaign of lies" against her by the president's personal lawyer, Rudy Giuliani.

At its core, the impeachment inquiry concerns Trump's July phone call with Zelenskiy that first came to attention when an anonymous government whistleblower filed a complaint.

In the phone conversation, Trump asked for a "favour," according to an account provided by the White House. He wanted an investigation of Democrats and 2020 rival Joe Biden. Later it was revealed that at the time the administration was withholding military aid from Ukraine.

"The bribe is to grant or withhold military assistance in return for a public statement of a fake investigation into the elections," Pelosi said. "That's bribery."

It's also spelled out in the Constitution as one of the possible grounds for impeachment -- "treason, bribery or other and high crimes and misdemeanours."

During Day One of the House hearings, career diplomats William Taylor and George Kent delivered sombre testimony about recent months.

They testified how an ambassador was fired, the new Ukraine government was confused and they discovered an "irregular channel" -- a shadow U.S. foreign policy orchestrated by Giuliani that raised alarms in diplomatic and national security circles.

It's a dramatic, complicated story, and the Democrats' challenge is to capture voter attention about the significance of Trump's interactions with a distant country.

With a hostile Russia its border, Ukraine is a young democracy relying on the U.S. as it reaches to the West.

Trump's reelection effort raised more than $3 million on the first day of public impeachment hearings, and campaign manager Brad Parscale announced it now hopes to raise $5 million within a 24-hour span.

A spokesman for the national Republican Party, Rick Gorka, said there's been a surge of volunteers and the response "we're receiving from the field has been tremendous."

Trump, who was set to headline a rally Thursday night in Louisiana, remained out of sight for most of the day and was monitoring a mass school shooting in California, according to aides.

Behind closed doors this week Pelosi reminded Democratic lawmakers of the importance of presenting a "common narrative" to the public as the proceedings push forward, according to a Democratic aide.

"We're in Chapter One of a process," said Rep. Raja Krishnamoorthi, D-Ill., a member of the Intelligence Committee conducting the inquiry. The challenge, he said, is educating Americans about what happened "and then explaining why it matters."

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Associated Press writers Jonathan Lemire, Laurie Kellman and Jill Colvin in Washington contributed to this report.