WASHINGTON -- President Barack Obama and challenger Mitt Romney declared a fleeting truce for partisan digs Tuesday as the nation remembered the 9/11 terrorist attacks, but campaign politics crackled through their somber observances.

The campaigns pulled their negative ads and scheduled no rallies. But both candidates stayed in the public eye as the nation marked the 11th anniversary of the jetliner crashes that left nearly 3,000 dead.

Obama observed a White House moment of silence, attended a memorial service at the Pentagon, visited Arlington National Cemetery and then met privately with wounded soldiers and their families at Walter Reed National Military Medical Center. But former president Bill Clinton carried on with a campaign stop for Obama in Florida, and the Democrat's camp issued registration appeals under first lady Michelle Obama's name.

In an echo of his usual campaign speech, Obama noted that the war in Iraq is over and troops are on track to leave Afghanistan in 2014.

"Al-Qaida's leadership has been devastated, and Osama bin Laden will never threaten us again," Obama said at the Pentagon. "Our country is safer and our people are resilient."

Romney, in Reno, Nev., to address a meeting of the National Guard, indirectly but clearly drew distinctions with Obama by spelling out his national security goals.

"I wish I could say the world is less dangerous now," he said.

After declaring that the day was not the proper moment to address differences with the president, Romney took issue with threatened cuts in defense and the handling of disability claims and called for more assertive international leadership.

"This century must be an American century," Romney said. "It is now our duty to steer it onto the path of freedom, peace and prosperity. America must lead the free world."

He alluded to his criticism of Obama over threatened cuts in military spending that would kick in if Congress and the president don't find agreement on major federal deficit reductions. While acknowledging that the war in Iraq is over and the U.S. is on a path to exit Afghanistan, Romney warned: "The return of our troops cannot and must not be used as an excuse to hollow out our military through devastating defense budget cuts."

Obama has insisted on a deficit deal that includes both spending cuts and increases in tax increases. Romney has blamed Obama for negotiating a deal that would require steep Pentagon cuts if a broad deficit agreement failed to materialize. But in an interview on NBC's "Meet the Press" last week, Romney said he also disagreed with Republicans who voted for that same deal. Among those was Romney's running mate, Rep. Paul Ryan of Wisconsin.

For Romney, the appearance before the National Guard also provided an opportunity to address men and women who have served in Iraq and Afghanistan. Romney has been criticized for not mentioning Afghanistan in his speech to the Republican National Convention two weeks ago. While Romney said Tuesday the U.S. goal should be to transfer security to Afghan forces in 2014 -- the same timeline as Obama's -- he cautioned, "We should evaluate conditions on the ground and solicit the best advice of our military commanders."

Obama's public appearances were largely spent in quiet contemplation, first during a moment of silence on the South Lawn at the White House at precisely the time that American Airlines Flight 11 became the first hijacked plane to hit the World Trade Center 11 years ago.

The first couple then attended a memorial and placed a wreath at the Pentagon, where another plane hit on Sept. 11. Afterward, they traveled to nearby Arlington National Cemetery to walk among the white headstones in a section devoted to the remains of those killed in Iraq and Afghanistan.

"This anniversary allows us to renew our faith that even the darkest night gives way to a brighter dawn," Obama said.

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.

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 was appearing at a rally for Obama in Miami, though campaign aides said he was likely to promote the president rather than criticize Romney. Clinton was to observe a 9/11 moment of silence.

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.

The Sept. 11, 2001, attacks killed nearly 3,000 in the United States and were followed by wars in Afghanistan and Iraq. At least 1,987 U.S. troops have died in Afghanistan and 4,475 in Iraq, according to the Pentagon. At least 1,059 more coalition troops have also died in the Afghanistan war and 318 in Iraq, according to iCasualties.org, an independent organization.

Polls show Obama leading Romney on terrorism and national security issues, but both issues are low priority for voters in an election dominated by the economy. A CBS News/New York Times poll conducted in July found 37 percent of voters called terrorism and security extremely important to their vote, while 54 percent said the economy and jobs were that important.