WASHINGTON (AFP) - A Democrat came close to outright victory in Tuesday's closely watched US congressional primary in Georgia, heading to a run-off in a race that Democrats tout as an early test of resistance to President Donald Trump.

Jon Ossoff, 30, came in first in a crowded field of candidates in a traditionally conservative 6th district, but narrowly fell short of passing the all-important 50 per cent threshold.

With 100 per cent of precincts reporting, Ossoff finished far ahead with 48.1 per cent support. The nearest Republican - former state secretary of state Karen Handel - came in at just 19.8 per cent.

Winning the June 20 run-off will be a steeper challenge for Ossoff, however, as Handel will almost certainly benefit from her party coalescing around a single candidate in a conservative-leaning district.

But Ossoff, a documentary filmmaker and former congressional aide, told energized supporters before all the returns were in that he and Democrats "shattered expectations" with their performance.

"There is no doubt that this is already a victory for the ages," Ossoff said.

"No matter what the outcome is tonight - whether we take it all or whether we fight on - we have defied the odds. We have shattered expectations."

As two candidates advance to a head-to-head vote Democrats still hope that Ossoff can capitalize on Trump's lackluster popularity and make the race a test of the president's first 100 days.

A shock upset in the national spotlight, the argument goes, would deeply embarrass the president and could jumpstart efforts to retake control of the House of Representatives in next year's midterm elections.

Georgia's 6th district is in the relatively affluent and conservative suburbs of Atlanta. It has remained a Republican fortress since 1978 when it was won by Newt Gingrich.

Ossoff is running in a special election there to replace congressman Tom Price, who resigned to become Trump's health secretary.

TRUMP WEIGHS IN

Under normal circumstances a Republican win would be in little doubt. But Trump's approval rating lags at around 40 per cent in a Gallup tracking poll - a record low for an incoming president.

A new Gallup poll shows just 45 per cent of Americans think Trump will keep his campaign promises, down from 62 per cent in early February.

The Ossoff political threat drew the personal interest of Trump, who recorded a robocall urging Republicans to troop to the polls and block the Democratic upstart.

Early on Wednesday, Trump weighed in again, claiming that a Republican finishing with less than 20 per cent support was a victory.

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

The Republican National Committee praised Handel's performance, with Chairwoman Ronna McDaniel congratulating her "for emerging from a large and qualified Republican field."

"These liberal Democrats failed to inspire voters," McDaniel said in a RNC statement.

In the run-up to Tuesday's vote, Ossoff had marshalled an army of volunteers, and reportedly amassed millions of dollars in out-of-state contributions by Democratic groups.

Liberal advocacy group MoveOn.org in a statement called Ossoff's first-place finish "a huge triumph for the Resistance and for progressives." It said: "The reason is clear: voters are rejecting Trump and his policies." .

GRASSROOTS ENERGY

Part of what is fueling Democratic excitement about the race is that while Trump won Georgia by six percentage points, the district that Ossoff seeks to win supported Trump by barely one point over Democrat Hillary Clinton.

It has many well-educated voters who are reliably Republican but frustrated by Trump.

The race quickly gained national attention, becoming the 11th most expensive election in House history, according to the Center for Responsive Politics.

The Atlanta Journal Constitution has reported that Republicans and Democrats have spent a combined US$14 million (S$19.5 million) on ad blitzes.

Ossoff is an untested candidate. He praises the grassroots organizers who he said are fueling support for his campaign and those of other Democrats considering mounting challenges in Trump-friendly territory.

To date Democrats have been unable to translate such energy into tangible election victories in the Trump era.

Last week, a Democrat challenging for an open congressional seat in a deep-red district in Kansas fell short of an upset.

The Democrat lost that race by seven percentage points, but the party sought to spin the result as a positive, pointing out that Trump won the district in November by 27 points.