US President Donald Trump has immediately distanced himself from a Republican candidate for governor crushed in a key election in Virginia.

In an electoral contest portrayed as one of the biggest tests of Mr Trump’s presidency since coming to office, Democrat Ralph Northam held a nine-point lead over Republican Ed Gillespie with 98 per cent of votes counted on Wednesday afternoon (AEDT).

The result was described by US media as a blow to Mr Trump and projected to be the biggest win for the Democrats in the crucial swing state since 1985.

Shortly before he gave a speech in South Korea, Mr Trump used Twitter to disown Mr Gillespie, a former Republican National Committee chair and GOP establishment figure.

Ed Gillespie worked hard but did not embrace me or what I stand for. Don’t forget, Republicans won 4 out of 4 House seats, and with the economy doing record numbers, we will continue to win, even bigger than before!

— Donald J. Trump (@realDonaldTrump) November 8, 2017

Mr Trump, who lost Virginia in the 2016 election to Hillary by five points, had backed Mr Gillespie in a series of tweets before polling day.

He also lent his support to Mr Gillespie via a robocall that was sent to Virginia voters, in which the President said: “Hello, this is President Donald Trump … I need you to vote for Ed Gillespie to be your new governor of Virginia.”

.@EdWGillespie will totally turn around the high crime and poor economic performance of VA. MS-13 and crime will be gone. Vote today, ASAP!

— Donald J. Trump (@realDonaldTrump) November 7, 2017

Mr Gillespie’s poor showing in the much-anticipated race prompted some observers to suggest the result reflected a wider backlash towards the President.

“As Norman Mailer called it, this is a switch of the cosmos, when everything just shifts,” said veteran political analyst Chris Matthews, a host on the cable station MSNBC.

“Voters have very crude tools to use to send a signal, basically, when there’s no presidential election. They’ve gone and send the signal they don’t like this president.”

Dogged by the ongoing investigation into Russian hacking and an inability to negotiate legislative wins on issues such as health care and tax, Mr Trump’s approval rating slumped to 37 per cent in a poll published by Washington Post-ABC News this week.

Though he was reluctant to completely embrace the President, Mr Gillespie took inspiration from Mr Trump’s combative campaigning style, particularly through the use of incendiary TV ads that hammered Mr Northam’s record on issues such as immigration and crime.

One ad linked Mr Northam’s voting record on immigration to a violent criminal gang, MS-13, that consists of people from El Salvador, Honduras and Guatemala, ABC reported.

GOP candidate Ed Gillespie (right) was defeated by the Democrat Ralph Northam. Photo: AP

Elections were also held in New Jersey, New York and Utah, but all eyes were on the Virginia race, held exactly one year after Mr Trump’s shock election win last November.

In the other major contest, Democrats reclaimed the New Jersey governorship, replacing Trump ally Chris Christie, who did not run again and will leave office with his approval ratings in the teens.

Exit polls supported the claim that Mr Trump had impacted the race, with 50 of voters saying the President was a major factor in their vote, according to CNN.

A swing state, Virginia is often considered a bellwether for presidential elections given its shifting demographics are reflective of wider national trends.

The outgoing Virginia governor Terry McAullife, a Democrat, has been tipped as a possible 2020 election candidate.