Vice President Joe Biden attended a memorial service in his native state of Pennsylvania, where one of the airliners crashed in the fields of Shanksville after passengers struck back at the hijackers. He told the families of the victims that "what they did for this country is still etched in the minds of not only you but millions of Americans forever."
Adapting a line from the poet William Butler Yeats, Biden said: "My personal prayer for all of you is that in every succeeding year you're able to sing more than you weep."
In Chicago, as he prepared to depart for Reno, Romney shook hands with firefighters at Chicago's O'Hare Airport, their yellow trucks forming a backdrop that recalled the sacrifice of first responders to the attacks in New York.
"On this most somber day, those who would attack us should know that we are united, one nation under God, in our determination to stop them and to stand tall for peace and freedom at home and across the world," Romney said in a written statement.
Romney's running mate, Wisconsin Rep. Paul Ryan spoke to an Air National Guard unit and at a fire station in his home state.
"This is a day we should be thinking about people who give their lives, and today what we are really thankful for are those who dedicate their lives to keeping us safe, for making our military, our Air Force, the unrivaled best in the world," he told about 80 members of 128th Air Refueling Wing at the Milwaukee airport.
To ensure at least a brief respite, both camps took their negative ads off the air, following precedent. A pro-Obama political group also withdrew its negative ads for the day. One pro-Romney group, American Crossroads, continued to air its anti-Obama ads.
One ad critical of Romney's stance on women's health care and abortion rights did appear on YouTube, the Internet video library, with a time stamp of Sept. 11, 2012. The Obama campaign said the ad is not new and was not airing on television Tuesday.
Obama and his allies have spent $188 million on TV commercials, according to information from media buyers provided to The Associated Press. Romney and the independent groups backing him have spent $245 million on ads through the end of August.
Ads or no ads, politics was unmistakably apparent throughout the day.
Clinton appeared at rally for Obama in Miami, where he contrasted the president's vision for health care, education and lowering the debt with that of Republicans.
He told the crowd that the best way people could honor those killed on 9/11 and those who served in Iraq and Afghanistan was to be a good citizen. "The least we can do is show up and vote," he said.
An interview with Obama, aired Tuesday on Miami's "The DJ Laz Morning Show," focused heavily on the election. White House aides said the interview was taped over the weekend while Obama was in Florida. The interview lacked any mention of the terrorist attacks.
Obama discussed immigration policy, education funding and Romney tax cut proposals that he said "would result in middle class taxes going up."
"We've got a lot of work to do, and I can only do it if I've got people out there who are getting registered and making sure they vote," Obama said.
Feller reported from Reno, Nev., and Kuhnhenn reported from Washington. Associated Press writers Stacy A. Anderson and Marks S. Smith in Washington, Sandra Chereb in Reno, Nev., and Julie Pace in Miami contributed to this report.
Follow Jim Kuhnhenn on Twitter at http://twitter.com/jkuhnhenn and Ben Feller at http://twitter.com/BenFellerDC