Polls show President Obama with a narrow but persistent lead over Mitt Romney as the campaign hits its final legs, and even Romney advisers are privately acknowledging that their candidate is the underdog.

But Romney still has a path to victory, with three debates in October that could change the race’s dynamics, Obama still under 50 percent in most polls and the economy still struggling.

Here are President Obama’s biggest challenges:

1. Turnout 

Much of the important work of the election will now happen out of public view, as each campaign tries to make sure their core supporters are properly registered and excited to vote. This is particularly important for the president.

The polls that show Obama ahead estimate that about a quarter of the electorate will be minority voters, a group that overwhelmingly favors the president. (If Obama wins, he is likely to carry more than 40 percent of the white vote, and 80 percent of the non-white vote.)

In 2008, about 26 percent of voters were non-white. But in 2010, when the Republicans made huge gains in the congressional elections, the minority share of the vote dipped to about 23 percent. Just as importantly, in 2008, 18 percent of voters were between the ages of 18 and 29, and two thirds of that bloc backed Obama. In 2010, those voters were only about 12 percent of the electorate, and only 55 percent of them backed Democrats.

Young and minority voters tend to stay home more during midterm elections and return during presidential years, so those drops are not surprising. But while much attention has been given to controversial voting laws in states across the country, Obama’s biggest challenge may be apathy.

Polls suggest that while African-Americans are as excited to vote as in 2008, young voters and Latinos are not. And the president needs all three groups. In 2008, he won North Carolina in part because he won young voters by almost 50 points, and they turned out in larger numbers than in 2004. Obama won Latinos in Florida by 15 points in 2008, while John Kerry, four years earlier, had lost among that key bloc.

2. The debates

The three debates between President Obama and Romney, as well as the one between Paul Ryan and Joe Biden, are perhaps the biggest events left of the election cycle. An estimated 52 million people watched the first debate between Obama and John McCain in 2008, an audience about 17 million bigger than Obama’s speech last week at the Democratic National Convention.

Related: 35 million watch Obama at DNC

The debates, and coverage of them by the press, will reach the critical undecided voters who will help decide the winner. And Romney has deftly used debates before: his sharp attacks against Rick Perry in one of Perry’s first debates helped weaken his potentially strongest challenger.

3. The Money

Republican groups and the Romney campaign have consistently raised more than Obama and his allies, although the president and his team raised the most in August. What this is likely to mean is huge spending by the GOP on negative ads in key battleground states, particularly highlighting the high rate of unemployment under Obama.

The effect these ads will have is completely unknown. The record $750 million raised by the Obama campaign in 2008 is likely to be dwarfed this year by both parties. If Republicans spend $1 billion and Democrats $950 million, it’s not clear that disparity would make a huge difference. Viewers in Virginia and Florida would see lots and lots of pro-Obama and anti-Obama commercials and could simply stop paying attention to them.

But a bigger spending gap could allow Romney to spend more money in more states and put Obama on the defensive.

4. The economy

The sluggish recovery (unemployment dropped to 8.1 percent only because more than 300,000 Americans left the workforce over the last month) is the constant opening for Romney. Nearly every poll shows voters like the president more than Romney, but voters seem wary of the president’s stewardship of the economy. As he did at the Democratic National Convention, the president must continue to make the case not simply that the economy is improving, but that he would lead an American recovery more effectively than Romney.

Related: Black unemployment, wealth still lag far behind

5. The unknown

Campaigns can often be influenced by completely unexpected factors. Republicans expected to win control of the U.S. Senate until their candidate in Missouri (Todd Aiken) made highly controversial comments about rape that have badly damaged the GOP. A surge in unemployment caused by the European economy could affect the election. Romney could gain traction on a surprising issue, like his recent and (false) attacks that have suggested the president would make it easier for people on welfare to get benefits without working.

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