ATLANTA — ONCE again, the youth vote went firmly Democratic in last week’s election — but the actual numbers, combined with some telling survey results, indicate that the party’s grip on the young may be loosening.

Six years ago, voters aged 18 to 29 favored Barack Obama over his Republican opponent, Senator John McCain, by a ratio of two to one and justified Time’s announcement that 2008 was “The Year of the Youth Vote.” Two years previously, in midterm races for the House, the same demographic went for Democrats 60 percent to 38 percent.

In 2012, President Obama’s advantage slipped, but under-30 voters still gave him a 23-point edge, 60 percent to 37 percent. No wonder this year the president appeared two days before the election at Temple University, where he exhorted the crowd, “So I need all of you to go grab your friends, grab your classmates... I need you to vote.”

But it turned out that 2012 was no anomaly. Turnout for young voters this year was around 21 percent, typical for midterms, but the breakdown was disappointing for the left: Exit poll data show that young voters backed House Democrats 54 percent to 43 percent, half the advantage of 2006 and two percentage points lower than in 2010.