Given the extraordinary attention paid to the campaign and Palin's surprise selection as John McCain's running mate, it stands a strong chance of becoming the most-watched vice presidential debate ever. The standard was the 56.7 million viewers in 1984, when Geraldine Ferraro was the first woman ever selected for a major party ticket.
Ifill, moderator of PBS' "Washington Week" and senior correspondent on "The NewsHour," is repeating her role from the 2004 debate between Dick Cheney and John Edwards. She had never done a debate before that, and admitted she was nervous about the large audience and all the people scrutinizing her performance.
The 2004 experience and work before a live audience during a recent swing of "Washington Week" shows done on the road have toughened her up.
"The biggest pressure you have as a journalist ever is to make sure you get an answer to your question," said Ifill, whose crowded resume includes The New York Times, The Washington Post and NBC News. "That's what I'm focusing on - how to ask questions that elicit answers instead of spin, or in this case to elicit engagement between the two."
The format offers Ifill great freedom. Questions on domestic or international issues are allowed, and it's up to her to decide the mix.
Colleagues suggest questions. So do viewers, people at her gym or folks she meets on the street. She politely takes them all, recognizing she has no monopoly on wisdom, but it doesn't necessarily mean she'll use them. Her goal is to help viewers learn something about the candidates they didn't know.
People sometimes forget it's a debate, not an inquisition, Ifill said.
"People who watch these debates are incredibly engaged," she said. "I don't have to chase the candidates around the table to make them answer questions. The people will know whether a question has been answered or not."
One competitor said he expects Ifill to do well. Bob Schieffer, CBS News chief Washington correspondent, said Ifill would be one of the people he recommends to succeed him as "Face the Nation" host.
"Gwen knows what she's doing," Schieffer said. "It's pretty hard to slip one past her. I think she'll do a great job."
There will be no shortage of armchair moderators. That's nothing new for Ifill, who a few weeks ago was involved in one of the more bizarre second-guessing episodes of the campaign.
PBS' ombudsman, Michael Getler, received some letters from viewers complaining about Ifill's facial expressions following Palin's speech to the Republican convention. Even though Ifill said that GOP delegates "exploded with excitement" over Palin's speech, those viewers apparently believed Ifill didn't seem sufficiently excited herself.
Huh? Her facial expression?
"In this blog-yourself age, we are all subject to that kind of scrutiny," Ifill said. "If you're frowning to think and you're sitting at a computer keyboard to write a piece for the AP, nobody sees you frown to think. If you're frowning to think after a long night on your feet on the (convention) floor with people around you, you're not even aware that your face is frowning to think. You're just thinking."
Partisans on both sides, sometimes in an organized fashion, are always ready to complain about things they don't like in the news, she said. With the Internet, they now have a greater opportunity to make those complaints public than they ever have.
Everything she says or does, everything she wears, every cocked eyebrow, is being interpreted, she said.
A thick skin is required. Ifill didn't go back to the tapes to look at her expression following Palin's speech.
"That would have required too much navel-gazing for me," she said. "I know where my head was so that's what counts."
PBS has tried to keep its coverage serious-minded. Throughout the party conventions, PBS showed much more of what was actually taking place than the commercial networks, which concentrated more on commentary. PBS reported the "lipstick on a pig" flap briefly but moved on.
"I don't have a problem with people covering it," she said. "It shouldn't be the only thing that people can find out about the campaign. If a voter calls up and says, `I want to know how the candidates feel about Social Security,' there should be a place where people can find it."
Except for a recent remark to her by a reporter at another network, Ifill has yet to hear it suggested that she favors Barack Obama for the presidency because they're both black.
"I have to admit that the reason I think it struck me so much is that I never get it - to my face," she said. "I never had anyone from either campaign suggest that I might be on one side because of race."
She challenges anyone to find anything in her work to suggest it.
"What you have to understand with all that criticism is that they have their jobs to do," she said. "You have your job to do. You have to be satisfied that you did a fair job."