Mike Singer had been highly critical of the US President in the days following violence in Charlottesville, when Mr Trump criticised "hatred, bigotry, violence - on many sides."
"The eyes of the country are on the Commonwealth of Virginia," Mr Singer tweeted. "As they should be.
"We are a wall against Trump and Trumpism."
"Virginia has told us to end the divisiveness, that we do not condone hatred and bigotry," Mr Northam said.
Continuing his tweets, Mr Singer said: "Let [Mr Northam's] win be not just a win for working Virginians but for decency and democracy themselves in a frightening time.
"I’m gonna go out on a limb and say this Virginia election is as historic, and as important, as Trump’s election itself."
He added: "If there was ever a question about what Charlottesville meant for Trump, tonight makes it clear: a fatal self-inflicted wound and a rocket for progress."
Mr Northam, the state's lieutenant governor, repeatedly sought during long months of divisive campaigning to tie Mr Gillespie to the US President.
His victory was in large part due to the surge in anti-Trump sentiment since the President took office.
Democrats reported record levels of enthusiasm heading into the race in Virginia, a swing-state and the only Southern state Mr Trump lost last year.
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