The incumbent Democratic mayor of Buffalo — running as a write-in candidate — declared victory on Tuesday night as he led a nearly 20 point lead over his Democratic Party opponent.

India Walton, a socialist backed by many high-profile progressives, refused to concede to Mayor Byron Brown in the highly-publicized contest until her campaign sees “all the votes,” her spokesman Jesse Myerson told The Post via text.

Brown, 63, who lost the June Democratic primary to Walton, claimed what would prove to be a stunning victory in an address to supporters shortly after 11 p.m.

“Today’s election, it’s not just a referendum on the future of the city of Buffalo, it was a referendum on the future of our democracy,” Brown said.

“At the very beginning they said we couldn’t win, they said it was impossible to win…You can never count a Buffalonian out!”

The race’s unofficial results showed Brown comfortably ahead of Walton, 39, with over 57,000 votes cast on election day and during the early voting period.

But mail in and military ballots won’t be counted until Nov. 16 — which means final results won’t be known for weeks still.

Long Island Democratic Rep. Tom Suozzi — who endorsed Brown — cheered what looked like a win.

“Byron Brown’s victory tonight is a clear triumph of core Democratic values over the far left socialist agenda. Voters know that pragmatism, solving problems and actually getting things done is what makes government work better,” he told The Post.

The incumbent also got the stamp of approval from the chairman of the state Republican Party Nick Langworthy — a former foe — who tweeted the early results indicate “socialism has been defeated in Buffalo.”

Brown, a moderate Democrat seeking his fifth term as an incumbent, is 16 years old. Walton, a nurse advocate and housing advocate, has never held public office.

Walton stunned the political world by defeating four-term incumbent Brown in a low-turnout June Democratic primary — the biggest upset since fellow Democratic socialist Rep. Alexandria Ocasio-Cortez toppled ex-Congressman and Queens Democratic Party leader Joe Crowley in the 2018 primary.

AOC stumped Walton during the campaign trail

Brown refused to leave the race and instead launched an online write-in campaign. According to a WIVB/Emerson College survey, Walton was led by Brown by 17 percentage points. 53.8 percent said they support Brown, while 36.2% of Buffalonians voted for Walton.

Meanwhile, Walton has attracted national attention, netting big name endorsements from the party’s left wing stars — including AOC, who headlined a rally with Walton in the weeks before the election and recently, former presidential candidate US Sen. Elizabeth Warren.

She’s also received backings from the party’s moderate leaders such as powerful US Senate Majority Leader Chuck Schumer and Sen. Kirsten Gillibrand.

However, some Democrats have made a point to stay out of the race — including Buffalo native, Gov. Jay Jacobs, the chairman of the state Democratic Party in Kentucky, and Kathy Hochul.

Jacobs has been asked repeatedly to explain the reasoning behind his refusal to back Walton — despite other Democrats such as the head of the county’s Democratic Committee and BOE Commissioner Jeremy Zellner endorsing her. He made a controversial comparison between Walton (a Black woman) and David Duke (ex-head of Ku Klux Klan).

Jacobs later apologised for his remarks after attracting backlash.