The Republican Party is to make its case next week for four more years of President Donald Trump after his Democratic opponent Joe Biden wrapped up his party's convention with a vow to end what he called a "chapter of American darkness."

Accepting the Democratic presidential nomination with an impassioned 25-minute speech, Biden said the United States was facing a "life-changing election" in November.

"The current president has cloaked America in darkness for much too long. Too much anger. Too much fear. Too much division," the former vice president said.

"May history be able to say that the end of this chapter of American darkness began here tonight as love and hope and light joined in the battle for the soul of the nation," the 77-year-old Biden said.

Trump, 74, also framed the contest in apocalyptic terms on Friday, accusing the Democrats of holding the "darkest, angriest and gloomiest convention in American history."

"Where Joe Biden sees American darkness, I see American greatness," he said.

Vice President Mike Pence also took aim at the four-day Democratic convention, where a parade of speakers denounced Trump's character, his handling of protests against racial injustice and accused him of mismanaging the coronavirus pandemic.

"So many of the speeches at the Democratic national convention were so negative," Pence said on "CBS This Morning." "They presented such a grim vision for America."

"Joe Biden last night said that democracy's on the ballot, that character's on the ballot The economy's on the ballot," Pence added.

The vice-president hailed an economic recovery whose "only real threat" was a Biden presidency.

The Republican convention begins in North Carolina on Monday and, like the Democratic event, it is going to be mostly online because of the COVID-19 outbreak which has left 175,000 people dead in the United States.

Trump is going to accept the Republican nomination in a speech at the White House on Thursday, shrugging off criticism over the use of the presidential residence for campaign purposes.  

- 'We must come together' -

Trump dismissed Biden's acceptance speech in a tweet.

"In 47 years, Joe did none of the things of which he now speaks. He will never change, just words!" Trump said, alluding to Biden's long record in politics as a senator from Delaware and Barack Obama's vice president for eight years.

Trump, who has been waging a campaign against mail-in voting, which polls show Democrats are more likely to use during the pandemic, also expressed concern that a result may not be known on election night.

"You're not going to be able to know the end of this election, in my opinion, for weeks, months, maybe never," he said.

Unlike four years ago, when Hillary Clinton emerged as the Democratic nominee after a bitter primary battle with Vermont Senator Bernie Sanders, the Democrats appear more united this time around.

Sanders, who also challenged Biden for the nomination and remains a favourite of the progressive wing of the party, has unabashedly thrown his support behind the nominee this time. 

"We must come together, defeat Donald Trump and elect Joe Biden and Kamala Harris as our next president and vice president," Sanders said. "My friends, the price of failure is just too great to imagine."

Also making the case for Biden at the party convention were three former presidents -- Jimmy Carter, Bill Clinton and Obama -- along with a host of Democratic luminaries and high-profile Republicans such as former secretary of state Colin Powell who are disenchanted with Trump.

Biden's White House run -- his third -- features a historic first with his choice of a black woman, California Senator Harris, to be his running mate.

Biden has been leading in the polls and Republicans will be looking to next week's convention to give Trump a bounce.

The latest RealClearPolitics average of national polls has Biden with a 7.4 point advantage over Trump and slight leads in several of the battleground states which are a key to victory in the Electoral College.

The candidates are scheduled to hold three debates -- the first on September 29 -- before Americans cast their ballots on November 3.