Tomorrow’s Democratic debate in Philadelphia potentially could be a disaster for Barack Obama — he must answer the toughest questions yet about his candidacy.

The event will mark the first time Senators Obama and Clinton have faced each other since February 26, their last debate in Ohio. That debate was before the Reverend Jeremiah Wright’s comments were made public and before Mr. Obama referred to millions of working class and small town voters as “bitter.” While Mrs. Clinton’s credibility has been marred since the last debate as well, she goes into it as the underdog and therefore has less to lose.

This debate could garner the most viewers of any of the debates — there have been more than 20 during the past year — according to a Northeastern University associate professor and expert on presidential debates, Alan Schroeder. Unlike most debates in this interminable election, which have been relegated to the backwaters of cable, tomorrow night’s discussion will be on network television, ABC, during prime time.

The timing and the circumstances make the debate difficult for Mr. Obama. Either of the ABC moderators, Charles Gibson and George Stephanopoulos, or Mrs. Clinton will have the opportunity to ask Mr. Obama about why the he has remained at his church and what exactly is it about bad economic times that cause some Americans to “cling” to religion.

The typical scenario for a front-running candidate like Mr. Obama is to sit back and run out the clock. Leaders in the polls don’t like to debate — an unscripted comment or mistake could alter the dynamic in a successful race. Yet passivity in the face of criticism about not being able to connect with middle America poses a greater risk for Mr. Obama.

While the junior senator is the favorite to capture the Democratic nomination, his odds for winning the Pennsylvania primary aren’t as good.

A lukewarm debate performance by Mr. Obama may increase Mrs. Clinton’s chances of winning the primary. A recent American Research Group poll suggests that she is ahead 20 points in Pennsylvania, a greater margin than the previous week’s survey, which had the candidates at a dead heat.

As Mr. Obama demonstrated with his stylistically-masterful speech on race in March, his set-piece orations are as good as any politician in recent American political history. While his ability to debate has improved, ad-libbing often fleshes out Mr. Obama’s prickly arrogance, such as when he sniffed to Mrs. Clinton that she was “likable enough” back in January. That comment drew little reaction other than serving to energize Mrs. Clinton’s feminist base. A display of superiority this time, however, will reinforce a perception of elitism of Mr. Obama, the exact aura he has been working to shed.

To have a successful debate Mrs. Clinton will need to do several things. She will, of course, need to win the primary in order to get a double-digit victory in Pennsylvania. She also needs to win over the undecided superdelegates, a key demographic of the debate. Although the math is not in her favor: Mr. Obama leads by 143 delegates.

A key swath of undecided independent voters will be seeing the candidates for the first time, among them the poorer Americans who don’t have cable television.

Therefore it is crucial for both candidates to make a good first impression on voters who likely will vote in the fall and who will be tuning into their first Democratic debate.

Mr. Gitell (gitell.com) is a contributing editor of The New York Sun.