WASHINGTON — Senator Mitch McConnell, the Republican leader, was not in a talking mood on Wednesday as he sped through the corridors of the Capitol. It had been roughly 12 hours since Doug Jones, a Democrat, had won election to the Senate — a victory that for Mr. McConnell spelled both bad news and good news.

“It was quite an impressive election,” Mr. McConnell, of Kentucky, said tersely. “It was a big turnout, and an unusual day.”

The election of Mr. Jones — the first Democrat to win a Senate seat in Alabama in 25 years — will have significant consequences on the national level, making it more difficult for Republicans to enact their legislative agenda and opening, for the first time, a possible — if narrow — path to a Democratic majority in the Senate in 2018. For Mr. McConnell, it has very immediate consequences: It reduces his already razor-thin majority in the Senate to 51 from 52 votes.

But it also relieves Mr. McConnell of a huge problem: having to deal with Roy S. Moore, the unpredictable Alabama Republican nominee whose candidacy was tarred by allegations that he molested a 14-year-old girl and made sexual advances toward other teenagers.