Republican Karen Handel won a high-stakes, closely-watched special congressional election Tuesday, salvaging a seat in traditional conservative Georgia where Democrats had hoped to strike a blow against Donald Trump's presidency.

Handel, an establishment Republican and former state secretary of state, defeated Democratic challenger and political novice Jon Ossoff by about five percentage points, multiple US networks projected, denying Democrats their first election victory of the year.

Her win marked an impressive rebound from polling that showed her narrowly trailing her rival in the run up to the vote, and signalled that Republican disillusionment about Trump was not as deep as Democrats were counting on.

The result no doubt comes as relief for Republicans who had grown concerned about whether their party, buffeted by the scandals that have plagued the Republican president, could hold the seat in Georgia's sixth district that became the most expensive congressional race in US history.

‘Congratulations to Karen Handel on her big win in Georgia 6th. Fantastic job, we are all very proud of you!’ tweeted Trump, who had publicly implored Republicans to troop to the polls to back Handel.

While the White House had played down the national importance of the Georgia race, Trump had gone all in on Handel, and the Republican Party no doubt sees the victory as a shot in the arm as it prepares to fight to preserve its control of Congress in next year's mid-term elections.

‘Tonight, the people of Georgia's sixth district overwhelmingly voted not only for Karen, but also for President Trump's agenda of replacing our broken health care system, reforming an outdated tax code, and prioritizing infrastructure investment,’ Republican National Committee chairwoman Ronna McDaniel.

- Trump gloats, Ossoff combative -

Trump's party also claimed victory in another congressional race Tuesday, in neighboring South Carolina. Both contests were held to replace lawmakers who vacated their seats in order to join the Trump cabinet.

Democratic candidates also lost earlier races in Republican-held districts in Kansas and Montana.

The victories gave Trump a chance to gloat. ‘Well, the Special Elections are over and those that want to MAKE AMERICA GREAT AGAIN are 5 and O! All the Fake News, all the money spent = 0,’ he tweeted.

The opposition Democrats had painted each of the races as a referendum on Trump's scandal-plagued presidency.

‘We dodged a bullet tonight,’ Georgia-based Republican strategist Chip Lake told AFP.

‘We're still going to be on the defense in November of 2018,’ with Trump a likely ‘liability,’ he added.

But while Republicans were facing a sobering reminder of their president's poor approval ratings, Handel steadied the ship in a district that only narrowly backed Trump in last November's vote.

In conceding the race, Ossoff, a filmmaker and former political staffer, signalled Democrats could learn from these races and prepare for the larger battle for control of the US Congress in 2018.

‘This is not the outcome many of us were hoping for,’ he told supporters. ‘But this is the beginning of something much bigger than us.’

Later on Twitter, Ossoff wrote that for months the district ‘has been a beacon of hope for people across the country. We showed the world that in places no one even thought it was possible to fight, we can fight. And we will fight.’

- Democrats need 'new message' -

At least one Democratic lawmaker called Ossoff's loss a ‘wake-up call for Democrats.’

‘Business as usual isn't working,’ congressman Seth Moulton said on Twitter.

‘We need a genuinely new message, a serious jobs plan that reaches all Americans, and a bigger tent not a smaller one,’ he added.

Ossoff tried to flip the Atlanta suburbs that Republican Tom Price left to become Trump's health secretary.

Republicans have held the seat since 1979. But as an increasingly well-educated, diverse suburban district it is the kind of territory that Democrats need to win if they want to gain the 24 seats necessary to reclaim the House in 2018.

Price won the district last November by 23 points, and Ossoff's much closer margin serves as a ‘silver lining,’ Democratic congresswoman Cheri Bustos said.

‘We're moving in the right direction,’ she told CNN.

A Handel win will undoubtedly energize Republican lawmakers in Washington, emboldening them as they seek to push forward with their controversial Obamacare repeal and tax reform legislation.