Obama, Romney campaigns suspend negative ads on 9/11 anniversary

President Obama and Mitt Romney are taking a rare break from the campaign cycle, suspending negative ads for a day and keeping their remarks apolitical out of respect for the victims of the Sept. 11 terror attacks -- as the nation marks 11 years since the tragedy.

Obama, first lady Michelle Obama and White House staffers observed a moment of silence Tuesday on the White House south lawn in memory of the Sept. 11, 2001, terrorist attacks.

The president and his wife then went to Arlington National Cemetery to visit the graves of service members killed in Afghanistan and Iraq.

The Obamas quietly walked between rows of graves at Section 60, which contains the remains of the most recent war dead. Pausing at several graves, Obama placed presidential "challenge" coins at the base of the headstones. The first headstone listed the names of 10 victims of an Oct. 26, 2009, helicopter crash in Afghanistan. They also placed a wreath at the Pentagon.

Later in the afternoon, Romney will speak at the annual conference of the National Guard Association of the United States, whose members were deployed as part of the U.S. response to the attacks.

Vice President Joe Biden will mark the day at a memorial service at Shanksville, Pa., where one of the hijacked airliners crashed after passengers fought the terrorists. Biden grew up in nearby Scranton, Pa.

In a dramatic turn-around from the two previous presidential elections after the 2001 attacks, the race this year has been dominated by the economy rather than national security concerns.

Polls show Obama leading Romney on terrorism and national security issues, but both are a low priority for voters concerned about sluggish economic growth and an unemployment rate still above 8 percent.

A CBS News/New York Times poll conducted in July found just 37 percent of voters called terrorism and security extremely important to their vote, while 54 percent said the economy and jobs were uppermost.

In 2004, the first presidential election after the Sept. 11, 2001, attacks, about two-thirds of voters said protecting the country was more important than creating jobs when deciding their vote for president, according to an AP-Ipsos poll conducted shortly before that election. President George W. Bush defeated Democratic challenger Sen. John Kerry in large part by convincing voters that he was the best candidate to keep the country safe.

That role now falls to incumbent Obama, who accepted the nomination for a second term at a Democratic convention that reminded voters at every turn that the president ordered the daring raid that killed Al Qaeda leader Usama bin Laden.

Attacks on the terrorist network continue. On Monday, an airstrike killed Al Qaeda's No. 2 leader in Yemen, Saeed al-Shihri, a Saudi national who fought in Afghanistan and spent six years at the U.S. military prison at Guantanamo Bay.

The post-Sept. 11 wars in Iraq and Afghanistan continue to have political implications. Romney did not mention Afghanistan in his speech accepting the Republican Party's presidential nomination, though he did address the war in a speech a day earlier. Still, Democrats criticized him for the omission from his nomination address.

The Associated Press contributed to this report.