In backing Roy Moore in Alabama's US Senate race even though the candidate faced allegations of sexual misconduct with teenage girls, President Donald Trump made a risky bet - and lost big.

The victory by Democrat Doug Jones over the Republican Moore in the Alabama special election on Tuesday was a catastrophe for Trump, portending a Democratic wave next year that could cost Republicans control of one or both houses of Congress.

The stakes in Alabama were that high. Democrats already were confident they had a strong chance to retake the US House of Representatives in next year's congressional elections. Jones' narrow victory will trim the Republicans' already narrow Senate majority to 51-49, opening the door for Democrats to possibly retake the chamber in next year's congressional elections.

"That Republicans lost in one of the most Republican states in the nation is a wake-up call no matter how flawed their candidate was," said Jesse Ferguson, a Democratic strategist and former aide to Democrat Hillary Clinton's presidential campaign.

Democrats never expected to have a chance in Alabama, where they had not won a US Senate race in 25 years. But the combination of Trump's unpopularity, the sexual misconduct allegations that erupted against Moore in November, and Trump's enthusiastic support of him anyway gave them the opportunity, experts said.

“Trump was the one who got Jones within firing range, and Moore allowed Jones to win,” said Kyle Kondik, a political analyst at the University of Virginia.

Even as Democrats lost several special congressional elections this year, they consistently showed higher levels of turnout and engagement, which is attributable to Trump, Kondik said.

TRUMP REFERENDUM?

The Alabama race showed there were limits both to Trump's endorsement power and his judgment.

Even as senior Republicans urged Trump to abandon Moore, the president decided instead in the campaign's final days to throw the full weight of his office behind him. In the end, that was not enough, and early turnout reports suggested that many Republicans stayed home.

Moreover, despite the sexual misconduct allegations against Moore, the race near the end increasingly seemed to become about the president. Moore's camp this week said the contest was specifically a referendum on Trump and his presidency.