Obama camp invokes memory of Gore's loss to motivate Dems

"Five-hundred thirty seven: the number of votes that changed the course of American history," a voice-over in the ad says. 

"The difference between what was and could have been. So this year, if you're thinking that your vote doesn't count, that it won't matter, well back then there were at least 537 people who felt the same way. Make your voice heard."

The ad, released Wednesday, follows a number of calls by the Obama campaign, including President Obama himself during stump speeches, urging voters to vote and, if possible, vote early.

The ad's release comes as both national polls and polls in key battleground states show a tight race. 

On Wednesday a Time magazine poll found Obama leading Romney by 5 percentage points in Ohio, a crucial battleground state for Obama's reelection chances. 

A national poll conducted by Gallup also found Romney leading Obama 50 percent to 47 percent among likely voters. Romney had previously led Obama by 5 and 7 percentage points over the last week.

The new ad airs in Colorado, Florida, Iowa, Ohio, New Hampshire, Nevada and Wisconsin.