The Donald Trump/Steve Bannon takeover of the Republican Party will have to be put on hold.

Tuesday’s Senate race in Alabama represented an attempt by the president and Mr. Bannon, his foremost political strategist, to show that they weren’t only in control of the party but could use their rebellious, antiestablishment message to drive their new version of the GOP to victory.

Instead, Roy Moore, the candidate Messrs. Trump and Bannon fully supported, lost in a state Republicans had controlled comfortably for most of the past two decades. Mr. Moore was a seriously flawed candidate, controversial enough to have been tossed off his own state’s Supreme Court twice, and more recently accused of having made improper sexual advances on teenage girls. Thus, much of the loss to Democrat Doug Jones will be laid at the candidate’s feet.

President Trump, center, with Sen. John Barrasso (R., Wyo.), left, and Senate Majority Leader Mitch McConnell (R., Ky.), right, on November 28. Photo: Ron Sachs/Zuma Press

Still, the loss is a huge blow to Mr. Trump personally. He now has backed three straight candidates for statewide office who have lost. He backed the loser in the Virginia governor’s race. He backed the loser in the Alabama Republican Senate runoff. And, in the past two weeks, he threw his full support behind the man who lost in the Alabama Senate general election.

The implications are enormous. If Mr. Trump’s message and personal power aren’t enough to win a state in the deep-red South, then mainstream Republicans will have little reason to think they can rely on those factors elsewhere. Nor will they think they are compelled to follow the lead of their own president on matters political.

That, in turn, will drive a further wedge between the president and Republican leaders—particularly Senate Majority Leader Mitch McConnell—who never wanted the party to throw its support behind Mr. Moore in the first place.

Similarly, Mr. Bannon’s pledge to field and fund nationalist, rabble-rousing Republicans to challenge a whole series of Republican senators up for re-election next year will strike less fear in the hearts of the party’s mainstream. His ability to act as a kind of Pied Piper leading a long string of candidates formed in his image to the front ranks of the party now is in doubt and will be resisted with new vigor by party regulars who fear he could lead the party to a broader disaster.

“The challenges the GOP faced remain and the finger pointing will only increase,” said Douglas Heye, a longtime top Republican congressional aide. “We remain bitterly divided.”

For Democrats, the victory in a state they never dreamed of winning just a few months ago delivers a jolt of energy—and, perhaps as important, could encourage balky donors who have left the party’s national machinery seriously underfunded this year. Heading into a crucial midterm election year, that combination of energy and dollars—and a belief in actually being able to win races in what last year was Trump country—is essential for Democratic hopes.

Perhaps most encouraging for Democrats, the same coalition of voters that propelled the party to victory in Virginia last month also emerged in Alabama. Exit polls showed that women made up a slight majority of the electorate and went for the Democratic Mr. Jones over the Republican Mr. Moore 58% to 41%. Nonwhite voters, principally African-Americans, made up one-third of the electorate and went for the Democrat 88% to 11%. And voters under the age of 30 went for Mr. Jones by a 60%-to-38% margin.

Democrats now will try to ride that combination of voters—women, minorities and young voters—to victory in race after race next year. They will need to pick off a net of only two Republican Senate seats to win the majority there, though that actually will be a tough assignment in light of the large number of Senate seats Democrats have to defend, several in GOP-leaning states.

Another key question Alabama’s results bring to the fore is whether Democrats can take control of the House of Representatives next year. That, too, remains an uphill climb.

The key numbers to keep in mind as Democrats approach that challenge are 24, 23 and 12.

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Democrats need to turn 24 House seats from Republican to Democrat to take control. Their best chance at winning that number starts with the 23 House seats held by Republicans from congressional districts that Hillary Clinton won in last year’s presidential election.

They will have to pick off most of those to win the House.

Meantime, they have to defend the seats of 12 Democratic House members from districts Mr. Trump carried last year.

In short, it’s still a tough task for Democrats in the contest that really will determine the contours of Washington in the era of Mr. Trump.

They will need a national wave. The results in Alabama certainly don’t guarantee such a wave. But they do suggest it’s possible.

Write to Gerald F. Seib at jerry.seib@wsj.com