With its upset win in Alabama, the Democratic Party has narrowed the Republicans' majority in the Senate to one seat - energising the party as it eyes the mid-term congressional elections 11 months from now.
Democrat Doug Jones, 63, a former prosecutor, beat controversial Republican Roy Moore, 70, for the Alabama Senate seat, which was considered a safe bet for the Republicans. This means the Republicans now have 51 seats to the Democrats' 49 in the Senate.
The mid-term elections next year offer a chance for the Democratic Party to wrest back Senate control, and put more legislative brakes on US President Donald Trump, who has been busy rolling back his predecessor Barack Obama's legacy.
"Now with 49 Democrats in the Senate, a path to a 2018 Democratic Senate majority is possible - still very tough given the seats up, but possible," tweeted Dr Larry Sabato, a professor of politics at the University of Virginia.
The special election for the seat vacated by Attorney-General Jeff Sessions morphed into a contest for Mr Trump and Mr Obama, as both waded in with appeals to voters in the final hours of the campaign.
Insisting that it was imperative for Republicans to keep the seat, Mr Trump stood by Mr Moore even as the former judge was accused of having made inappropriate sexual advances towards a teenage girl four decades ago.
Mr Trump tweeted: "Doug Jones is Pro-Abortion, weak on Crime, Military and Illegal Immigration, Bad for Gun Owners and Veterans and against the WALL. Roy Moore will always vote with us... VOTE ROY MOORE!"
Mr Obama called Mr Jones "a fighter for equality, for progress". He added: "Doug will be our champion for justice."
Mr Jones' victory marked the first time that the Democrats have won the seat in 25 years.
This time, college-going and suburban white Americans seemed to have voted Democrat, outweighing Mr Moore's support from evangelicals as well as blue-collar and rural white Americans.
Indications are that African Americans turned out in numbers to vote for the Democratic candidate.
"We don't yet know if they reacted to the racism, anti-Muslim sentiments, or misogyny. But it does seem that Alabama voters responded against something that had gone too far," said Dr R. Marie Griffith, a professor of religion and politics at Washington University in St Louis, Missouri.
"It's huge for a Democrat to win a Senate seat in Alabama. It is a sign of what may be changing in the country," added Dr Griffith, author of a new book, Moral Combat: How Sex Divided American Christians & Fractured American Politics.
The result was seen as a slap in the face for Mr Trump and his former chief strategist Steve Bannon, who had campaigned stridently for Mr Moore. For one thing, Mr Trump's endorsement did not work.
Preliminary data shows Mr Moore lost 12 counties that Mr Trump won last year.
"Trump's base is not sufficient to carry an election," Dr Glenn Altschuler, a professor of American studies at Cornell University, told The Straits Times.
In an e-mailed comment, Texas Democratic Party chair Gilberto Hinojosa wrote: "Donald Trump and Republicans embraced a known racist and alleged paedophile, and the people said 'hell no'."
He was referring to allegations that Mr Moore had inappropriately touched a 14-year-old girl when he was an assistant district attorney in his early 30s. He has also been accused of having habitually prowled a mall looking for high school girls. Several women came out to support the allegations, but Mr Moore denied them, calling them a political conspiracy.
Several Republicans in Washington were openly uncomfortable with supporting Mr Moore, but the party rallied around him. Tuesday night showed that this may have been a mistake, as the contest turned into a question of morality in a strongly evangelical state.
Added Dr Altschuler: "In terms of the issue of Republicans tacitly or explicitly endorsing a person who was credibly accused of molesting a 14 year old, this is not going to go away."
A possible turning point for Mr Moore was when senior Alabama Senator Richard Shelby, a Republican himself, told CNN on Sunday: "I think, so many accusations... when it got to the 14-year-old's story, that was enough for me. I said I can't vote for Roy Moore."