Charles Gibson, moderator of tonight's Democratic debate Charles Gibson, moderator of tonight's Democratic debate

On the eve of April 22's Pennsylvania primary, Senators Barack Obama and Hillary Clinton will clash in a 90-minute Democratic presidential candidate debate from Philadelphia (tonight at 8 pm/ET, ABC). At ringside is Charles Gibson, the ABC World News Tonight anchor who shares moderating duties with George Stephanopoulos. We caught up with Gibson to get his thoughts on the grueling campaign.

TV Guide: Explain the significance of Obama vs. Clinton.
Charles Gibson: It's self-evident — we have two candidacies that I'd never thought I'd see. It is the first time since 1928 that there hasn't been a president or a vice president on the ticket for one party or the other. It's the first time we've had anything this wide open in my lifetime.

TV Guide: What are the keys to the knockout for Obama?
ibson: He has to continue what he's doing. He has a nice lead in delegates and if he doesn't get behind in the popular vote he's OK.

TV Guide: And for Clinton?
Gibson: If she wins Pennsylvania, she probably has to win North Carolina, and that's a much bigger hurdle. She may have to sweep the big ones that are left.

TV Guide: McCain's almost been forgotten.
Gibson: He's got a totally clean field, so he has time to put money in the coffers. It also allows him time to distinguish himself and plot strategy.

TV Guide: Do you think viewers can reach debate overload?
Gibson: As one of the candidates said to me, "I can do my opponent's lines and my opponent can do my lines, and I'm not sure anyone would notice." You have to find ways to make the debate different.

TV Guide: How does this campaign compare to others you've covered?
Gibson: I don't think I've ever seen a primary season go on this long. I have no idea how this gets resolved. There's no kingmaker.

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