In the southern US state of Virginia, a Republican candidate has been assured of victory over his Democratic rival in a closely-contested gubernatorial election, dealing a heavy blow to President Joe Biden.

The poll on Tuesday has been widely viewed as a bellwether ahead of next year's mid-term elections.

With 99 percent of votes counted, Republican Glenn Youngkin was leading Democratic incumbent Terry McAuliffe by 2 points.

US major media outlets reported on Wednesday that Youngkin was most likely to win.

Former President Donald Trump expressed his support for Youngkin. However, the former businessperson has kept a distance from Trump during the campaign, garnering support from non-affiliated voters.

Virginia was leaning toward the Democrats in recent years. President Biden easily won the state in last year's presidential race with a margin of 10 points over Trump.

Experts say President Biden's dwindling approval ratings have affected the governor's election. It appears the Biden administration has been handed a severe verdict, heading into the mid-terms.