MOUNTAIN BROOK, Ala. — For Republicans, it did not have to come to this.

After a fraught campaign that tested the country’s tolerance for political sideshows, Roy S. Moore, the party’s Senate nominee, lost to the Democrat Doug Jones in a special election on Tuesday. Even before the race, Republican leaders in Alabama and Washington had taken to describing Mr. Moore as a kind of biblical plague — a punishing force inflicted on them, and outside their control.

But for all his local fame and notoriety, Mr. Moore was never an inevitability in the Senate race. Nor was it inevitable that Alabama would hold a Senate election this year at all, let alone one that the Democrats would win.

To grasp the sheer improbability of the election unfolding here, consider the improbable — and improbably ill-fated — decisions Republican leaders made to bring things to this point.