An upstart New York progressive on Wednesday appeared poised to oust a veteran congressman in what would be a major score for the US Democratic Party's far left.

Jamaal Bowman, a black 44-year-old, looked set to topple Eliot Engel, a 73-year-old white centrist, with 60.76 percent of the vote versus his rival's 34.37 percent, according to preliminary numbers from the New York Board of Elections.

Final results might not come in for days or even weeks, with many ballots sent in to the coronavirus pandemic. But Bowman's lead in the district that covers parts of the Bronx and Westchester, north of Manhattan, appeared insurmountable.

The closely watched race looked likely to end in an upset in the style of Alexandria Ocasio-Cortez, who two years ago dealt a stunning blow to veteran Joe Crowley, nabbing the seat in Congress he had maintained for two decades.

Engel, current chairman of the House Foreign Affairs Committee, has spent some three decades in Congress and received endorsements from the establishment, including from Hillary Clinton, top House Democrat Nancy Pelosi, Senate minority leader Chuck Schumer and New York Governor Andrew Cuomo.

But he faced a stiff challenge from Bowman, a former middle school principal, who rode a wave of support from some of the party's most outspoken progressives, including Ocasio-Cortez -- who sealed re-election in a neighbouring New York district -- along with Senators Bernie Sanders and Elizabeth Warren.

The winner of the primary in the heavily Democratic district is all but assured a seat in Congress. Bowman ran on a platform including Medicare for All, a Green New Deal, and a focus on racial and economic justice.

As anti-racism protests flared in recent weeks he called to defund law enforcement and repeal the 1994 crime bill, legislation passed under former president Bill Clinton that critics hold was detrimental to communities of colour and lead to increased mass incarceration.