A young Jewish Democrat, Jon Ossoff, has given US President Donald Trump a fright by taking a huge share of the vote in a congressional by-election.

Mr Ossoff this week won 48.1 per cent of the ballots in the fight for a traditionally rock-solid Republican district in suburban Atlanta.

Although he failed to win the 50 per cent necessary to win the seat outright, Mr Ossoff, a documentary film-maker, was claiming victory this week.

His nearest rival, former Georgia secretary of state Karen Handel, managed 19.8 per cent, and will stand against Mr Ossoff in a run-off on June 20.

Mr Ossoff, 30, the only Democrat in a field of 11 Republicans, was targeted by Mr Trump on Twitter.

Mr Trump derided the result and said that the run-off proved the Democrat could not win.

Mr Ossoff’s campaign slogan was “Flip the 6th” because the election was being run in Georgia’s traditionally Republican Sixth District.

The poll had been called because Mr Trump had named Congressman Tom Price, a Republican, as his Health Secretary.

“This is already a remarkable victory,” Mr Ossoff told the New York Times. “We defied the odds, shattered expectations, and now are ready to fight on and win in June.”