LEARN WHAT IT IS. >> WE'RE JUST HOURS AWAY FROM THE FIRST EVER PRESIDENTIAL DEBATE BETWEEN VICE PRESIDENT KAMALA HARRIS AND FORMER PRESIDENT DONALD TRUMP. ABC NEWS WILL HOST THE DEBATE AND ABC'S CHRISTIANE CORDERO IS LIVE IN PHILADELPHIA AT THE NATIONAL CONSTITUTION CENTER, WHERE TONIGHT'S DEBATE WILL TAKE PLACE. CHRISTIANE >> JOVANA. PHILIP, GOOD TO BE WITH YOU BOTH. YOU KNOW, MORE THAN JUST A BEAUTIFUL BACKDROP, THE NATIONAL CONSTITUTION CENTER IS ALSO A FITTING PLACE TO LAY OUT THE VISION FOR THE NATION'S FUTURE. THESE TWO CANDIDATES, THEY HAVE SPARRED BEFORE, FROM AFAR, BUT NEVER SIDE BY SIDE AND NEVER WITH STAKES AS HIGH AS TODAY. FOR THE FIRST TIME EVER, VICE PRESIDENT KAMALA HARRIS AND FORMER PRESIDENT DONALD TRUMP WILL STEP INTO THE SPOTLIGHT. A MERE SIX FEET APART AND WITH A RAZOR THIN GAP IN THE LATEST POLLS. >> I THINK THIS IS A VERY EXCITING AND NERVE WRACKING, AND THERE'S A LOT OF TENSION IN THIS COUNTRY. >> I THINK IT'S GOING TO BE INTERESTING TO WATCH. I'M INTERESTED TO HEAR WHAT BOTH INDIVIDUALS HAVE TO SAY. >> THEY EACH HAVE TWO MINUTES TO ANSWER EACH QUESTION, TWO MINUTES FOR REBUTTAL, AND AN ADDITIONAL ONE MINUTE FOR A FOLLOW UP CLARIFICATION OR RESPONSE. NO AUDIENCE, NO CAMPAIGN STAFF ALLOWED IN THE ROOM. ABC'S DAVID MUIR AND LINSEY DAVIS WILL MODERATE THE DEBATE WHILE SITTING BELOW A COUNTDOWN CLOCK FACING THE CANDIDATES. TO PREPARE, HARRIS HAS SPENT THE PAST SEVERAL DAYS IN A PITTSBURGH HOTEL RUNNING MOCK DEBATES UNDER FULL STUDIO LIGHTING, BRIEFLY STEPPING IN AND OUT OF THE CAMPAIGN TRAIL TO PUSH HER ECONOMIC AGENDA AND PUSH BACK AGAINST HER OPPONENT. >> THERE IS NO FLOOR FOR HIM IN TERMS OF HOW LOW HE WILL GO, AND WE SHOULD BE PREPARED FOR THAT. WE SHOULD BE PREPARED FOR THE FACT THAT HE IS NOT BURDENED BY TELLING THE TRUTH. >> TRUMP HAS NOT HELD MOCK DEBATES, INSTEAD MEETING WITH ADVISERS AND GOING AFTER HARRIS'S RECORD AS VICE PRESIDENT IN THE BIDEN ADMINISTRATION. >> SHE WILL BE THE WORST PRESIDENT. HE IS THE WORST PRESIDENT IN HISTORY. SHE WILL BE WORSE THAN HIM. >> AS HE RALLIED VOTERS IN WISCONSIN OVER THE WEEKEND, DAYS BEFORE HEADING TO PHILADELPHIA. >> IS ANYBODY GOING TO BE WATCHING? >> TRUMP IS SCHEDULED TO ARRIVE IN PHILLY TONIGHT. HARRIS GOT IN LAST NIGHT. NOW NO OPENING STATEMENTS TO KICK OFF THE DEBATE. THERE WILL BE CLOSING STATEMENTS. EACH CANDIDATE GETS TWO MINUTES. THE ORDER WAS DECIDED BY A VIRTUAL COIN TOSS. TRUMP WON AND OPTED TO HAVE