Charlie Neibergall/AP Photo

HEMPSTEAD, N.Y. - Exactly three weeks before voters head to the polls, President Obama and Republican nominee Mitt Romney face-off here today for the second presidential debate that could again chart a new course for the general election campaign.

In an email to supporters Monday, Obama for the first time declared the race "tied," conceding that a lead he held ahead of the first debate has shrunk or disappeared. Romney meanwhile has been touting what he calls a "growing crescendo of enthusiasm" for his campaign.

Tonight's debate - a town hall style showdown - could provide an opportunity for each candidate to gain the edge he desires.

Here are five things to watch tonight that could shape voters' perceptions before the final debate next Monday:

A More 'Aggressive' Obama: Obama is expected to compensate for his lackluster showing in the first debate with a more aggressive approach to Romney and attempt to hold him accountable for his past positions on key issues. Obama is also likely to come armed with some "zingers" - those snappy one-liners which he largely avoided in the first debate but has deployed in a steady stream against Romney on the campaign trail since.

The new strategy is clearly aimed at reassuring Democratic voters of his passion for the job and willingness to fight for the values that helped get him there.

But could it backfire? It is generally more difficult to go on attack in a town hall setting, where the focus is on each undecided voter who poses a question to the candidates. Obama will have to walk a fine line of appearing presidential and not petty.

"You should expect that he's going to be firm but respectful in correcting the record and the times we expect Mitt Romney will hide from and distort his own policies," said Obama campaign spokeswoman Jen Psaki. "He's energized and I expect he will also be making a passionate case."

Romney Striving for Empathy: Romney has struggled this campaign to convince voters of his ability to understand their struggles, a challenge he will confront again at tonight's town hall when faced with questions from the audience.

A majority of registered voters - 51 percent - in the latest ABC News/Washington Post poll say it's Obama who better understands their economic problems, compared to 42 percent for Romney. The president also leads his rival on who is more friendly and likable (58 to 32 percent) and who is more honest and trustworthy (55 to 47 percent).

Faced with a personal query from an undecided voter, Romney has a chance to showcase his empathetic and understanding side, and score points with millions of voters watching at home. He has much more practice this campaign cycle doing answering voters' random questions, attending 23 town hall meetings compared to Obama's single such forum, which was held in July. Romney's most recent town hall meeting was last week.

Body Language: Obama and Romney will perch atop tall chairs and use handheld microphones, rather than stand behind podiums, in the second debate. They will also have the ability to stand, roam the stage and work the room.

Their bodies will be in full view during Q&A creating the opportunity for viewers to study their posture, mannerisms and body language for presidential poise as they answer questions and while listening to their opponent. In first debate, Romney looked at Obama expressionless during his responses, and was praised for his disciplined behavior. Obama often looked down and his podium and was caught smirking at times.

Experts say style matters, particularly in the town hall. Obama campaign aides say the president will be more energetic and lively. The Romney camp says Mitt will be Mitt. It will be worth watching closely how the approaches play in the new setting and whether either candidate ventures into the other's personal space.

Meddling Moderator? Moderator Candy Crowley of CNN has drawn preemptive fire from both campaigns for saying publicly that she will do what a moderator does: ask follow-up questions at tonight's debate. Her probing could shape the course of the conversation by steering the candidates from their straight-up talking points and holding them accountable for what has, or has not, been said.

"When something comes up that maybe could use a little further explanation, might be as simple as 'But the question, sir, was oranges and you said apples. Could you answer oranges?'" Crowley said of her role Monday in an interview on CNN.

Crowley has dismissed grumbling from both sides and the suggestion that she should be relegated to the role of, essentially, a glorified host. She says moderators of past presidential town halls, including in 2004 and 2008, helped in "furthering the discussion" by asking additional questions and directing the discussion after an audience member's initial question. It's worth watching how she makes her mark.

Wild Card Questions: Town hall meetings are notorious for those out-of-the-blue questions that can put the candidates in an awkward place and for sparking some tense and testy exchanges. "What's your favorite girl scout cookie?" one woman asked Obama in Cincinnati in July. "How can we trust either of you with our money when both parties got us into this global economic crisis?" a man asked Obama and Sen. John McCain at the town hall debate in 2008.

"We have no idea what they're going to come up with," said Gallup pollster Frank Newport, whose organization is responsible for picking the random sample of uncommitted voters to attend the debate.

"It's the one time we let average people come up with questions. As a pollster is fascinating to hear what is on the minds of voters," Newport told ABC News.

The audience will consist of roughly 80 Nassau County, N.Y., residents representing a diverse cross-section of society, officials said. The attendees will write their questions upon arriving at the debate hall and submit them to Crowley, who will make the selections.

ABC News' Russell Goldman contributed to this report.