A great unanswered issue may decide whether President Barack Obama can win a second four years in office.
In 2008, young Americans owned the presidential battle. They tweeted and watched the YouTube clips that propelled Obama into office. They signed up, unequivocally, for Hope and Change. The pollster John Zogby says the under-30s cast 19 per cent of votes that year, up from 17 per cent in 2004. Their turnout rate was 52 per cent, the highest since 18-year-olds got the vote in 1972.
The outcome? While older Americans were more or less evenly divided between the two candidates, the millions of "extra" young voters broke for Obama, by a margin of two to one. They were, in short, the first black President's margin of success.
This year, things are different.
The kids who bought into "Yes We Can" have run out of sunny optimism. And who can blame them? The unemployment rate for under-25s is 13 per cent. More than four million people under 30 are out of work. Many have huge debts: the average university student graduates US$27,000 ($33,000) in the red, a figure rising by about 5 per cent a year. Obama has only marginally improved their prospects.
A growing number of under-30s fall into a group that Zogby calls "cengas": people who are college educated and not going anywhere.
"These kids are increasingly jaded and increasingly libertarian, in that they have no confidence in government to solve their problems," he said.
"Obama still has the black vote, he still has the Hispanic vote, and the creative classes. But young voters are the one part of his core constituency still equivocating."
As a result, his support among under-30s is down by about 10 per cent on 2008. The only saving grace, from the President's point of view, is that, for all their frustration, most deserters are yet to migrate to the Romney camp. Instead they are calling themselves "undecided".
"There are currently double digits in that column," Zogby said. "The libertarians among them may be rejecting Obama, but they are just as bothered by the Republican Party's invasions of privacy on social issues. So as things stand, it's likely they're just not going to vote. To win, Obama needs to change that, and persuade them to turn out."
Romney also has problems exciting his base. About 10 per cent of white evangelicals are currently failing to back him, according to Zogby. (One-third of them cite his Mormon faith as the reason.) These voters are also unlikely to vote, Zogby believes. In a way, he said, the race now revolved around two candidates who on paper should not win, "even though one of them has to".
Foreign policy - the subject of today's debate - is an area where Obama may strike a natural chord with young voters. While Romney often seems to base his view of the world on the idea of American "exceptionalism", a pitch more likely to appeal to older viewers, the President sees America as a global citizen. That's more in line with that of the twentysomethings who came of age in a connected world.
If all that fails, the Democrats have one other trump card. The prospect of a Romney White House may be enough to scare equivocating young voters to the ballot box.
The coming days will see America's airwaves flooded with hundreds of millions of dollars' worth of negative adverts.
They will come from both sides, of course. But Obama's will be pitched firmly at ambivalent twentysomethings, highlighting Romney's opposition to abortion and gay marriage, and portraying him as a socially conservative elitist.
It will not be pretty, but after four difficult years, a man who won office because of hope is likely to seek re-election through fear.