By Yael T. Abouhalkah, Kansas City Star Editorial Page columnist

Tuesday's official exit polls are coming in just a few hours. Here's what they could mean -- and not mean -- for supporters of Barack Obama and John McCain.

This is the first presidential contest since the disaster of 2004, when exit polls predicted a 3-percentage point victory for John Kerry over George Bush.

Pollsters involved in exit interviews on Tuesday say they will avoid the mistakes -- especially the early release of exit poll data -- that occurred in 2004.

A good Washington Post story outlining how the polls are supposed to work this year can be found here.

But even when the polls come out, take into account that millions of people have already voted in advance in more than two dozen states. So they won't be counted in the exit polls. Did more Obama supporters take advantage of advance voting? Or McCain backers?

In addition, remember that GOP voters -- especially older Americans -- don't care to participate in the exit polls. That helps explain the undercounts of Republican support for candidates in some exit polls, such as Bush in 2004.

Finally, no one knows what to expect of younger voters who are going to the polls in 2008 for the first time. Will they show up in large numbers predicted by Obama's backers?

Or will the young voters stay home -- and essentially not show up in the exit polls? If that happens, McCain will get a boost in the exit polls.