President Obama enjoyed a historic advantage among young voters in 2008, but this year, he isn’t likely to win those voters by as wide a margin. What’s more, young voters’ share of the electorate could drop from where it’s been during the last four presidential elections.

(Mark Duncan/AP)

A new Harvard Institute of Politics national poll of 18-to-29-year-olds likeliest to vote shows Obama leading Mitt Romney 55 percent to 36 percent. In 2008, the president won young voters by a whopping 34-point margin -- 66 percent to 32 percent -- over Sen. John McCain (R-Ariz.). 

The poll -- which was conducted Sept. 19-Oct. 3 -- also shows that Obama has improved his standing among young voters since March, albeit slightly. In March, Obama led Romney 51 percent to 34 percent.   

Political observers and even casual followers of politics have likely found themselves talking about the importance of the "youth vote" at some point or another. To understand the clout young voters wield in the electorate, it's worth looking at what did and did not change in 2008, compared to previous elections. 

As The Fix boss noted earlier this year, the youth vote -- which refers to voters 18 to 29 --  is often misunderstood. Take 2008, when talk about the strength of Obama's support from young voters was everywhere. Here's what happened that year: Obama won young voters by a wider margin than any Democratic nominee in the last three decades.

What didn't happen is often overlooked. Young voters' share of the electorate barely changed in 2008 -- it ticked up a single percentage point (to 18 percent) from where it stood during the three previous elections. In other words, from 1996-2004, fewer than one-in-five people casting presidential ballots were 18 to 29. In 2008, that was still true. 

So for Obama in 2012, it's less about boosting turnout across the board than it is about keeping his share of the youth vote high. 

That said, there are signs that young voter turnout may not even be where it was in 2008 and the previous three presidential elections. In combined Washington Post-ABC News polls in September and October, 67 percent of registered voters under 30 said they are absolutely certain they will vote. During the same period in 2008, that number was 80 percent. 

A recent Pew Research Center study indicated that that just 63 percent of young registered voters said they definitely planned to vote -- a drop from 72 percent four years ago. The Pew survey showed that only about half of adults under 30 said they are absolutely certain they are registered to vote -- also a drop from 2008. 

The Harvard poll showed that young voters favoring Romney are more likely to turn out than young voters supporting Obama. Put simply, young Romney supporters are less common than young Obama supporters, but they appear to be more resolute. Sixty-five percent of young voters supporting Romney say they will definitely vote this fall, compared to 55 percent of young Obama supporters. 

It's clear that Obama will win young voters by double-digits in November, and the Harvard poll shows they trust the president more than Romney on major policy issues. But it's looking increasingly likely that the president won't win young voters as decisively as he did in 2008. 

This isn't that surprising. In 2008, votes were yearning for change and Obama was a fresh face with a natural knack for connecting with the young. Four years later, he is an incumbent presiding over difficult economic times. 

Less clear is what impact a drop-off among young voters will have for Obama in the swing states where both he and Romney have competed so heavily.  A lot depends on how close the margins are there, and what percentage of a given state's electorate young votes will account for.