US Secretary of State Mike Pompeo and First Lady Melania Trump address the Republican convention on Tuesday after a first night that featured depictions of a United States under a Joe Biden presidency that veered between the apocalyptic and the hyperbolic.

Republicans had pledged to provide an uplifting convention to celebrate Donald Trump's nomination for four more years in the White House but the mood at the outset was mostly somber.

Kimberly Guilfoyle, 51, a former Fox News host and the girlfriend of Donald Trump Jr, described the coming election as a "battle for the soul of America" -- echoing a phrase former vice president Biden has used throughout his campaign.

"They want to destroy this country and everything we have fought for and hold dear," Guilfoyle said. "They want to steal your liberty, your freedom."

Don Jr framed the November election between his 74-year-old father and "radical left" Biden, 77, as a choice between "church, work and school" and "rioting, looting and vandalism."

The coronavirus pandemic which has left more than 177,000 people dead in the United States and dented Trump's reelection hopes was largely ignored although Don Jr claimed there is "light at the end of the tunnel."

Trump, who is trailing the Democrat Biden in the national polls, continued his attack meanwhile on mail-in voting, which is expected to play a larger role than in past elections because of COVID-19.

"For our Country to be sending 80 million UNSOLICITED BALLOTS is very unfair and a roadmap to disaster," Trump tweeted. "Fraudulent & missing Ballots like never seen before."

Opponents say Trump's resistance to expanded mail-in voting -- a method already used widely in the United States -- is an attempt to suppress voter turnout, while setting up an excuse to challenge the result if he is defeated.

Besides Pompeo and Melania Trump, two of Trump's other children, Eric Trump and Tiffany Trump, will be among the featured speakers as Republicans present the case for the president's reelection.

"President Trump has ensured the safety of America -- and SECURED our many FREEDOMS, which is the cornerstone of this great nation," Pompeo said in a tweet.

"I look forward to sharing with you how the President has delivered on that mission TONIGHT!" said Pompeo, who will deliver pre-recorded remarks from Israel, where he is making an official visit.

'Political prop'

An appearance at a party convention is unusual for a top US diplomat and the Biden campaign accused Pompeo of "blatant use of his office for overtly political purposes."

"Secretary Pompeo's decision to serve as an errand boy for the President's reelection on a taxpayer-funded diplomatic mission, and his decision to use one of our closest partners as a political prop in the process, is absolutely disgraceful," Biden's deputy campaign manager Kate Bedingfield said.

According to the Israeli media, Pompeo may deliver his remarks from the roof of the King David Hotel in Jerusalem, providing a backdrop of the Holy City that may appeal to Trump's evangelical supporters.

Melania Trump, 50, will speak live from the White House gardens and will be hoping to avoid the controversy which surrounded her 2016 convention speech, when she was accused of lifting passages from the remarks of another first lady -- Michelle Obama in 2008.

Like last week's Democratic convention, most of the Republican event is being held online because of the COVID-19 pandemic.

Trump on Monday did visit Charlotte, the North Carolina city where the convention is being held, to make his acceptance speech and he is also expected to attend his wife's remarks on Tuesday.

As Republicans kicked off their convention on Monday, a high-profile former party chairman, Michael Steele, announced that he was joining The Lincoln Project, a group of prominent Republicans dedicated to Trump's removal.

His move came as more than two dozen former Republican members of Congress, including ex-Arizona senator Jeff Flake, threw their support behind Biden.

"After the turmoil of the past four years, we need a president who unifies rather than divides," Flake said. "We need a president who summons our better angels, not a president who appeals to our baser instincts.

"That's why we need Joe Biden."