US President Joe Biden speaks at the dedication of the Dodd Centre for Human Rights at the University of Connecticut on October 15. Photo: AFP

 US President Joe Biden on Tuesday threw his weight behind the struggling Democratic candidate in Virginia's closely watched election for governor, urging a crowd to defeat Republican "extremism."

Standing by Terry McAuliffe, whose slide in the polls has put Democrats on red alert for a wider Republican comeback in 2022's midterm battle for Congress, Biden mocked the Republican opponent as an "acolyte of Donald Trump."

"You've had the courage and the wisdom to reject the extremism that has taken over the Republican Party all across America," Biden told the crowd in Arlington, a Virginia city in the Democrat-heavy suburbs of Washington.

"Today's Republican Party stands for nothing but just keep cutting taxes for the wealthy and the most powerful corporations." 

On paper, McAuliffe, 64, should have a relatively easy run in a week's time against Republican Glenn Youngkin.

Already a former Virginia governor and seeking to succeed the outgoing Democrat, McAuliffe is the closest thing to an incumbent in a state where Biden trounced Trump in the presidential election 12 months ago.

Instead, Youngkin is tied with McAuliffe in the polls, eying an upset victory on November 2 that would energize his party's push to regain control of Congress in the 2022 midterms.

Democrats are deploying all their big guns.

Popular former president Barack Obama has already campaigned for McAuliffe and Vice President Kamala Harris is headed to Virginia on Friday.

"Vote, vote, vote," Biden said. "Show up for democracy, for Virginia, for the US of America."

Part of the problem for the Democrats in Virginia and across the country is simply cyclical. They can expect to lose Congress in 2022's midterms because that is almost always what happens to the party of the sitting president.