WASHINGTON -- Haven't registered to vote? Do it now.
Haven't gotten around to updating your voter registration? Do it now.
Aren't absolutely sure you're registered correctly? Reregister now.
Tell everyone else to do it, too.
And consider today your deadline.
Those are the core messages behind National Voter Registration Day, a broad-based, nonpartisan effort to turn Tuesday, Sept. 25, into a national day of action and push people to register to vote before it's too late for the November election.
"As a basic organizing principle, having a date as a deadline seems to work really well," said Dan McSwain, a spokesman for the coalition of more than 1,000 groups.
According to the coalition, 6 million Americans didn't vote in 2008 because they missed a registration deadline or didn't know how to register. State deadlines vary widely, but if you register today, you're definitely in time.
And everything would-be voters need to start the process is available online, at the National Voter Registration Day website. There are also events being held across the country and celebrity spokespeople.
"If you are registered to vote, the best thing you can do is share it on social media," McSwain said. "We're asking everyone to share this as much as possible."
Blogging on HuffPost on Monday, Elisabeth MacNamara, president of the League of Women Voters of the United States, noted that 24 percent of Americans who are eligible to vote are not registered, but close to 75 percent of those registered to vote will actually cast a ballot.
"Imagine what our elections and country might look like if we did a better job of registering more voters," she wrote. "Voter registration is the key to getting Americans participating in the political process."
MacNamara's suggestion for National Voter Registration Day: "Take a picture of yourself with your voter registration application or wearing an 'I'm registered' sticker, and post it on Facebook or Twitter in order to encourage your friends to get registered."
In some states, the voter registration process can be done entirely online, but in most, after filling out the information on the National Voter Registration Day website, you'll need to print out something, sign it and mail it in.
A recent survey found that more than half of the young people who were first eligible to cast a presidential ballot in 2008 do not know today whether they are registered to vote at their current address.
McSwain encouraged people who aren't absolutely sure of their registration status to reregister at their current address. "It can't hurt. It doesn't break any rule," he said.
On HuffPost Live on Monday, Heather Smith, president of Rock the Vote, said, "If we all talk about voter registration together, hopefully everyone around this country knows that you have to get registered. If you've moved, if you've changed your address, if you've changed your name, you have to get registered -- and you have to do that now so you're prepared for the election."