2017-04-19 14:24:02 UTC
Jon Ossoff fell short of the 50% needed to win outright in a congressional vote in Georgia and there will be a run-off.
Mr Ossoff ran on a Make Trump Furious ticket in a suburban Atlanta seat that has been Republican since the 1970s.
Mr Trump took a keen interest in the race and said the result was a "BIG 'R' win".
The seat was made vacant when Congressman Tom Price left to join the Trump administration as US health secretary.
Mr Ossoff won 48.1% of the vote with all 210 precincts reporting.
The Democratic Party candidate will now face Republican Karen Handel, a former Georgia secretary of state, in the 20 June run-off for the seat in Georgia's 6th Congressional District.
She came a distant second with 19.8%.
There were 11 Republicans among 18 candidates in the first-round primary, which had divided conservative support.
Mr Ossoff said: "This is already a victory for the ages. We have defied the odds, we have shattered expectations."
The grassroots movement aiming to topple President Trump
Republicans believe the run-off will favour them in a seat they have held since 1979, when it was won by Newt Gingrich, but the Democrats have spent a large amount of money on the campaign and will press hard for victory.
Mr Ossoff, a 30-year-old political newcomer, most recently worked as a documentary filmmaker after serving as a congressional aide to Georgia representative John Lewis.
Despite his lack of experience, he raised a whopping $8.3m (£6.5m) in the first quarter, making it one of the most expensive congressional races ever.
Mr Trump tweeted: "Despite major outside money, FAKE media support and eleven Republican candidates, BIG 'R' win with runoff in Georgia. Glad to be of help!"
Analysis: Rajini Vaidyanathan, BBC News
Jack Miller, 15, campaigns for Republican Karen HandelImage copyrightREUTERS
Jack Miller, 15, campaigns for Republican Karen Handel
Democrats in Georgia will be celebrating, and commiserating tonight.
On one hand, Jon Ossoff achieved a big victory. With very little political experience he came out top in a solidly safe Republican seat. But he didn't win big enough to take the fast lane to Congress and needs to win the run-off.
Republicans - including the president, who has been following this closely - will be also be celebrating and commiserating. While they didn't lose control of the 6th district, this seat should never have been in contention. They will be hoping their chances will be much stronger now they only have one, instead of 11 candidates, on the ballot.
Karen Handel has experience and name recognition. But, in a race which is being framed as a referendum on the Trump presidency, will she align or distance herself from him?
In November's presidential race, Mr Trump only beat Hillary Clinton in this district by the tiniest of margins.
Republicans were spared humiliation last week when they narrowly defended a deeply conservative Kansas seat vacated when Mr Trump appointed Congressman Mike Pompeo to lead the CIA.
A Democrat majority win on Tuesday would have been a major embarrassment for President Trump and could have spurred Republican lawmakers in competitive districts elsewhere in the country to distance themselves from him ahead of the nationwide 2018 mid-term elections.
Sensing the danger, Mr Trump had attacked Mr Ossoff in tweets on Tuesday morning as a "super Liberal Democrat" who was "very weak on crime and illegal immigration, bad for jobs and wants higher taxes".