President Barack Obama and Republican challenger Mitt Romney pulled their negative ads and avoided appearing at campaign rallies in honor of the 11th anniversary of the terrorist strike. But the day offered Romney a chance in a speech to a meeting of the National Guard to address criticism that he didn’t include a salute to the troops in his convention speech. Obama’s camp sent former President Bill Clinton to swing-state Florida for an evening rally eight weeks before Election Day.

The president and first lady Michelle Obama observed the anniversary with a moment of silence on the White House’s South Lawn at 8:46 a.m., the time that American Airlines Flight 11 became the first hijacked plane to hit the World Trade Center. They stood side by side, heads bowed, as a bell tolled three times, then watched with their right hands over their hearts as a bugler played taps.

The Obamas then rode through the Washington area strewn with flags at half-staff to the Pentagon, the target of another of the four planes hijacked by al-Qaida operatives. Aided by a Marine honor guard, Obama placed a white floral wreath near a concrete slab etched with the date and time that another of the hijacked airplanes struck the building before observing another moment of silence.

“Eleven times, we have paused in remembrance, in reflection, in unity and in purpose,” Obama told the crowd of family members of the victims. “This is never an easy day.”

Afterward, Obama shook hands with the Pentagon crowd, including a man in a Romney campaign hat.

The president then went to Arlington National Cemetery, where he visited the graves of recent war dead from Afghanistan and Iraq and placed presidential challenge coins in front of their headstones. He later planned to visit wounded soldiers and their families at Walter Reed National Military Medical Center.

At the time of the somber White House observance, Romney was shaking 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. The Republican nominee was flying to Nevada to address the National Guard, whose members deployed as part of the military response. The speech was an opportunity for Romney to address criticism from Democrats that he’s not ready to be commander in chief — criticism that was silenced for the day but sure to continue in the coming weeks.

“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.

Vice President Joe Biden attended a memorial service in his home state of Pennsylvania, where one of the hijacked airliners crashed in the fields of Shanksville. 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.”

Romney’s running mate, Rep. Paul Ryan of Wisconsin, planned to spend the day in his home state and hadn’t scheduled any public events. Ryan said in his own statement that Sept. 11 is a time to pay tribute to those who quietly work to prevent attacks and to those in the military “who have sacrificed so much, including their lives, for the same end.”

The attack killed nearly 3,000 in the United States and was 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.

Tracking civilian deaths is much more difficult. According to the U.N., 13,057 civilians were killed in the Afghan conflict between 2007, when the U.N. began keeping statistics, and the first half of 2012. Going back to the U.S.-led invasion in 2001, most estimates put the number of Afghan deaths in the war at more than 20,000. Estimates vary widely in Iraq, but most agree that at least 100,000 Iraqis were killed in war-related violence in the years between the invasion in 2003 and the U.S. withdrawal last December.

Perhaps the most obvious signal that the presidential campaign is on hold is that negative ads will be taken off the air, following precedent. 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.

Polls show Obama leading Romney on terrorism and national security issues, but both are a 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.

Obama’s campaign says it still sees an opportunity to focus on national security and terrorism in the final weeks of the campaign. National security issues resonate particularly well in battleground states with large military and veteran populations, namely Virginia, North Carolina and Florida. Obama’s campaign has been running TV ads in those states focused on the president’s policies for veterans, and surrogates have held national security-focused events there as well.

In 2004, the first presidential election after the 9/11 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 Associated Press-Ipsos poll conducted shortly before the election. President George W. Bush defeated Democratic challenger 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 nomination for a second term at a Democratic convention that reminded voters at every turn that U.S. forces killed al-Qaida leader Osama bin Laden on Obama’s watch.

The post-9/11 wars continue to have political implications. Romney did not mention Afghanistan in his speech accepting the GOP’s presidential nomination. While he had spoken about the war a day earlier to the American Legion, his critics were quick to note that he had not mentioned the ongoing conflict and the troops fighting in it.