Bernie Sanders said Wednesday night that what Democrats "need in this campaign is energy. We need youth." And if he is to have any real chance of winning the Democratic nomination and the presidency, he's right -- especially on his second point.

Our December Washington Post/ABC National poll showed Sanders just barely edging out Hillary Clinton among Democratic-leaning registered voters under 40 years old, 45-44. But among older voters, Clinton trounced Sanders, carrying the 40-64 age bracket 66-19. Young voters are often easier to attract to rallies during the campaign than to the polls on Election Day; older people are much more reliable voters.

In that sense, Sanders is trying to replicate the enthusiasm among young people generated by President Obama in 2008 and 2012. Eight years ago, Obama took 66 percent of the under-30 vote in the general election and he did nearly as well in 2012. Obama also beat Clinton among young voters in the 2008 primaries. A combined sample of NBC News exit polls on Super Tuesday that year showed Obama up 16 points over Clinton.

But if Sanders can get those younger voters to turn out in the primaries, they could prove useful in the general election. A December Quinnipiac poll found 18-to-34 year-olds favoring Sanders over both Sen. Marco Rubio (R-Fla.) and Sen. Ted Cruz (R-Tex.) by 29 points each. Clinton led Rubio by 28 points in the same age group, and Cruz by 22.

The Post-ABC poll was conducted Dec. 10-13 among a random national sample of 1,002 adults reached on conventional and cellular telephones. Overall results have a margin of sampling error of plus or minus 3.5 percentage points. The error margin is plus or minus 5.5 points among the sample of 377 Democrats and Democratic-leaning registered voters.