A young Democrat, Jon Ossoff, in the US state of Georgia has come very close to winning congressional seat held by Republicans for decades.

Ossoff won 48 percent of Tuesday's vote in Atlanta's 6th District and will go into a runoff vote in June with his Republican rival Karen Handel.

The district has been a Republican stronghold for almost 40 years.

The election has been closely watched throughout the country and is widely considered to be an early popularity test for President Donald Trump .

Republican Senator Lindsey Graham of South Carolina said the race showed how the South was changing.

"This should be a wake-up call for the Republican Party in the South," he told NBC's "Today" show.

Ossoff, a former congressional staffer and documentary filmmaker, ran on a pledge to "Make Trump Furious".

Trump jumped into the race in its final days with attacks on Twitter poking fun at Ossoff for not living in his own district and for attracting support from Hollywood celebrities.

The president weighed in again on Wednesday.

Despite major outside money, FAKE media support and eleven Republican candidates, BIG "R" win with runoff in Georgia. Glad to be of help!

— Donald J. Trump (@realDonaldTrump) April 19, 2017

But among Democrats, there is optimism that Ossoff's campaign may provide a blueprint for future races, with midterm elections coming up next year.

"We are changing the world and your voices are going to ring out across this state and country," Ossoff told his supporters.

"There is no amount of dark money that will overcome grassroots energy, so bring it on."