More than 60 million Americans have already cast ballots ahead of the November 3 election at a record-breaking pace that could lead to the highest United States voter turnout by percentage in more than a century.

Brisk early voting has led Michael McDonald, the University of Florida professor who administers the US Elections Project, to predict a record voter turnout of about 150 million, representing 65 per cent of those eligible to vote, the highest rate since 1908.

US voters already have cast more early votes during this presidential campaign — 60.5 million — than they did in all of 2016 when they passed the 47 million mark earlier this month, data shows.

With just eight days to go until the election, US President Donald Trump on Monday wooed voters in Pennsylvania, a battleground state he won in 2016 that is crucial to his chances of defeating Democratic challenger Joe Biden and winning a second term. A state whose voters can swing toward either major party, Pennsylvania has been heavily courted with frequent visits by both candidates.

Trump, a Republican, addressed a rally in Allentown and was due to speak later in the day in Lititz and Martinsburg.

He planned multiple trips to Michigan, Pennsylvania and Wisconsin this week, as well as visits to Arizona, Nebraska and Nevada.

“I think we're going to win Pennsylvania by than more than we did last time,” Trump told reporters before the Allentown rally.

His 2016 victory in Pennsylvania was vital in his win over Democrat Hillary Clinton.

Addressing supporters, Trump touched on what he called “an existential” issue for Pennsylvania, Biden's comment during last week's presidential debate that if elected he would “transition” the US away from oil and natural gas.

“He wants to go with windmills that are made in Germany and China,” Trump said, adding, “Biden's plan is an economic death sentence for Pennsylvania's energy sector.”

Biden was expected to remain in his home state of Delaware on Monday. He is scheduled to travel to Georgia on Tuesday, with stops in Atlanta and Warm Springs, a small town where Democratic President Franklin D. Roosevelt, who took office during the Great Depression and rolled out his New Deal recovery programme, died in 1945.

Despite Biden's solid lead in national opinion polls, the contest in the most critical battleground states that could decide the outcome appears tighter. Biden will be aided with an appearance in Orlando, Florida, on Tuesday by former President Barack Obama, under whom he served as vice president.

Biden's push in Georgia, a state that went for Trump in 2016 by about five percentage points and has not backed a Democratic presidential candidate since 1992, shows Biden's effort to expand his party's reach as polls show a competitive race in the state.

Kremlin criticises Biden

Meanwhile, The Kremlin criticised Biden on Monday, saying his assessment of Russia as the biggest threat to US national security was wrong and encouraged hatred of Russia. Biden named Russia as the most serious global threat to the US in an interview with CBS's “60 Minutes” broadcast on Sunday, while calling China the biggest competitor.

American intelligence officials said in August that Russia was employing “a range of measures” to interfere in the presidential race to denigrate Biden and help Trump.

Covid-19 blame game

Surging coronavirus cases in many parts of the country and a Covid-19 outbreak within Vice President Mike Pence's staff have kept the focus of the race on the pandemic. Pence, due to campaign in Minnesota on Monday after an appearance in North Carolina on Sunday, tested negative for the coronavirus on Monday, his office said, after multiple senior aides tested positive over the weekend.

Record numbers of new US Covid-19 cases in recent days have offered Biden's campaign a chance to remind voters of how Trump and his allies have played down the advice of public health experts to wear masks and observe social-distancing guidelines to battle the pandemic.

Biden on Sunday accused Trump of surrendering to the virus that has killed about 225,000 people in the country.

Trump lashed out at Biden on Monday, calling him “a pathetic candidate” while again asserting that his administration was “doing a great job” against the pandemic and saying again that the US was “absolutely rounding the corner".

White House Chief of Staff Mark Meadows said on Sunday that the administration was “not going to control the pandemic,” and was instead focused on vaccines and therapeutics.

The outbreak among the vice president's staff marked the latest White House Covid-19 cases, which have included Trump, first lady Melania Trump, their son, Barron, and numerous aides and associates.