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When will we know a winner on election night?

One piece of advice for anyone planning to tune in to election night coverage Tuesday: "Have a couple of cups of coffee," CBS News director of elections Anthony Salvanto said. It's going to be a long night.

Though polls in some states, like the key battleground of Virginia, will close at 7 p.m. EDT, others, including the critical swing state of Iowa, won't close until 10 p.m. EDT - "and most of the battlegrounds may even still be counting votes at that point," Salvanto said.

"I don't think we'll know the winner there for a while, especially if it ends up as close as it looks right now - that could be hours," he said, predicting only a one- or two-point gap between President Obama and Mitt Romney across battleground states. But as returns come in throughout the night, Salvanto added, results from "battlegrounds within the battlegrounds" - crucial voting blocs in swing states - could sketch a rough outline of how the election will likely turn out.

Citing Hispanics, African-Americans, women, seniors in Florida, and suburban white men, Salvanto said watching the way those groups are breaking "will give us indications of maybe which way this is leaning. It'll generate some excitement."

In 2008, for example, typically conservative small towns and rural areas defected, making up a critical part of then-candidate Obama's coalition. Recent polling, Salvanto said, suggests those areas may shift back to Romney on Tuesday, meaning the president "would have to make up that difference somewhere else," like around cities or among women.

Above all else, though, the number one thing to watch for Tuesday night: "Turnout," Salvanto said.

"When you see an election that is this close, with each partisan side in their respective camps and so few undecided voters, it becomes about which candidate gets their folks out to the polls," he continued. "And people often say, 'Oh it's all about turnout.' But in this one, it really is."

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