One year since first debate: Clinton vs. Obama

— -- Illinois Sen. Barack Obama and New York Sen. Hillary Rodham Clinton will meet Wednesday at National Constitution Center in Philadelphia for the 24th debate or forum of the Democratic presidential campaign. ABC will broadcast the faceoff from 8 to 10 p.m. ET. It's been almost a year since the Democratic candidates first debated last April at South Carolina State University. Then, eight presidential hopefuls crowded the stage. That number has dwindled to a pair of finalists. USA TODAY examines the debates and the outlook for Wednesday:

How important have they been?

The debates have been one of several factors — including fundraising, big speeches, retail campaigning and TV ads — that have winnowed the Democratic field and defined the race. Gaffes have occasionally put candidates on the defensive. Debates have spotlighted Clinton's command of policy and have televised Obama's persuasive abilities unfiltered to a wide audience. "I don't think an Obama could have happened … absent the Internet and absent this kind of exposure in debates," says Allan Louden, who teaches political communication at Wake Forest University in North Carolina.

What each candidate needs to do

Terry Madonna, director of the Franklin & Marshall College Poll in Pennsylvania, says this debate could be unusually influential because of high interest and volatility in the electorate. Polls show Clinton's once double-digit lead in Pennsylvania has narrowed.

In the debate, Obama presumably will try to calm the furor over his comments on voters in the Rust Belt feeling "bitter."

And Clinton? "She needs to shake things up," Madonna says.

Obama's worst moment

At the first debate in April 2007, Obama was asked what he would do if two American cities were attacked by terrorists. He said he would make sure emergency services were adequate and then would turn to the question of intelligence findings, avoiding alienating the world community with a response that might be "based on faulty intelligence, based on bluster and bombast." In contrast, Clinton said she would "move as swiftly as is prudent to retaliate."