The Democratic and Republican presidential nominees, in a statement, said they will appear together at the World Trade Center site on Thursday "to honor the memory of each and every American who died" in the 2001 attacks.
The campaigns already had agreed to suspend television advertising critical of each other on Sept. 11. The McCain campaign has said it will air no ads that day.
Both campaigns have been running negative television ads and, at the just-concluded political conventions, pulled no punches in exploiting partisan differences.
Obama and McCain said Thursday will be different.
"All of us came together on 9/11 - not as Democrats or Republicans - but as Americans," they said. "We were united as one American family. On Thursday, we will put aside politics and come together to renew that unity."
A group backing community service, MyGoodDeed.org, wants Sept. 11 to become a national day of voluntary service and had asked that Obama and McCain perform acts of community service instead of campaigning.
Nearly 3,000 people were killed after hijackers rammed passenger airplanes into the Twin Towers in New York and the Pentagon. The death total includes 40 passengers and crew members aboard the fourth hijacked plane, United 93. It crashed in a field near Shanksville, Pa., as passengers rushed the cockpit, investigators believe.