A closely watched special congressional election in Pennsylvania remained nail-bitingly close early on Wednesday -- bad news for nervous Republicans who saw President Donald Trump romp to victory there in 2016 and face mid-term elections in November.
With more than 99% of precincts reporting in southwestern Pennsylvania's 18th congressional district, young Democratic candidate Conor Lamb declared victory in a speech to cheering supporters.
"It took a little longer than we thought but we did it!" he said.
There was no immediate response from his Republican rival Rick Saccone, a Trump ally, or any official announcement of results.
CNN said later that with all precincts reporting, Lamb was ahead by just 0.2 percentage points -- 49.8% to 49.6%.
The race was so close that the results from outstanding absentee ballots would likely decide the outcome.
A Lamb victory would not endanger Republican control of the House of Representatives -- the Republicans currently have 238 seats to 193 for the Democrats, with four vacant.
But it would be seen as a stark warning that districts which Trump won in 2016 could now be for grabs in November's vote.
The election, seen by many as a referendum on Trump and the Republican Party, was taking place in a working-class district that Trump won by about 20 points, suggesting the latest developments were good news for the opposition party.
With the race in the balance, Saccone, whose campaign drew criticism from Republican strategists for its lacklustre performance in recent weeks, told his followers earlier that "It's not over yet."
"We're not giving up," Saccone added.
The National Republican Congressional Committee also refused to concede.
"This race is too close to call and we're ready to ensure that every legal vote is counted. Once they are, we're confident Rick Saccone will be the newest Republican member of Congress," it said in a statement.
But the performance by Lamb provided the broader takeaway.
"It's a win for the Democrats even if they don't actually win this," Kevan Yenerall, a political science professor at Clarion University in Pennsylvania, told AFP.
For Republicans, it sets off alarm bells.
"If you're a Republican and you're in a marginal, competitive district... you're going to be concerned," he added.
The 33-year-old Lamb, a former federal prosecutor and US Marine officer, ran a surprisingly strong race against conservative state representative Saccone, 60.
His performance in a district held by Republicans for the last several elections has the potential to shake up the political landscape on a national level.
And it would reassure Democrats they can win in what has been considered Trump country, giving them a surge in enthusiasm, fundraising and grassroots activism as they head toward the all-important mid-term elections.
Veteran political analyst Stuart Rothenberg said it was notable that in a district long dominated by Republicans, the party's voters "either stayed home or flipped to Lamb."
Who wins the district was immaterial, Rothenberg argued on Twitter. "The outcome is clear."
Trump had endorsed Saccone and made an 11th-hour campaign stop with the candidate.
But Lamb's strong performance underscored the challenges that Republicans face as they prepare to defend their majority in both the Senate and House of Representatives in November.
The party occupying the Oval Office tends to lose seats in the first mid-terms after winning the presidency.
With chaos swirling in the White House, Trump's approval rating under water, and a special prosecutor investigating potential collusion between Trump's campaign and Russia, Democrats are counting on a domino effect at the polls to send them back to power in Congress.
Republican organisations have poured millions of dollars of outside money into the race in the hopes of dragging Saccone across the finish line and signaling that their position ahead of the mid-terms is not as precarious as Democrats suggest.
Democrats have noted that Lamb's blue-collar roots, support of organised labour and moderate political views were helping him win crossover support in conservative areas.
In District 18, several of Trump's campaign promises have resonated with voters -- in particular, his call for restricting immigration, supporting gun rights, and bringing back jobs in the coal and steel industries.
Lamb has worked to avoid alienating Trump supporters, and downplayed the idea that the Pennsylvania election is a referendum on the controversial commander-in-chief.
The district's voters "overwhelmingly want me to work with the president," Lamb said after casting his ballot on Tuesday.
Lamb was endorsed by the main mineworkers and steelworkers unions in the region, whereas Saccone has had a tempestuous relationship with organised labour.
Democrats came close to flipping Republican seats several times in US House special elections last year, in Georgia, Kansas, Montana and South Carolina. While those Democrats ultimately fell short, they fared better than Hillary Clinton did against Trump in those districts in the 2016 presidential race.
Pennsylvania 18's previous congressman, Republican Tim Murphy, resigned last year amid an adultery scandal.