* Romney and Obama both withhold television ads
* Campaigns walk political tightrope
* Obama visits Pentagon, Biden to Shanksville
By Samuel P. Jacobs
WASHINGTON, Sept 10 (Reuters) - The killing of Osama bin Laden and the war in Afghanistan are becoming increasingly charged campaign issues, but both U.S. presidential candidates will put politics on hold to observe the anniversary of the Sept. 11 attacks on Tuesday.
President Barack Obama and Republican nominee Mitt Romney will freeze their barrage of television ads for the day a s ceremonies across the United States remember the 2,977 people killed by the 2001 attacks. It is recognition of the awkwardness of throwing political punches on a day when politicians typically call for the nation to come together.
"Both campaigns have to be very, very careful to appear respectful and avoid the impression of exploiting a national tragedy for political purposes," said Bill Galston, a former adviser to President Bill Clinton and fellow at the Brookings Institute.
Although there will be no overt campaigning, in such a tight race the two men and their aides always have one eye on messaging that can move polls in their favor.
Romney may be the candidate who most needs to shore up his national security credentials, both as the challenger and because he was criticized even by senior Republicans for omitting to mention the war in Afghanistan during his acceptance speech at the Republican convention last month.
On Tuesday morning, Obama and first lady Michele Obama will join a moment of silence on the South Lawn at the White House and then appear at a memorial service at the Pentagon. Obama will also visit with wounded soldiers and their families at Walter Reed National Military Medical Center.
Romney plans to address the National Guard Association Convention in Reno, Nevada, and campaign staffers in at least six states will collect donations and organize a letter-writing campaign for U.S. troops serving overseas.
Romney's wife, Ann, is expected to participate in events marking the anniversary in Florida. Romney's campaign will halt operations in Virginia, home to the Pentagon where 184 people were killed in the al Qaeda attacks.
"Team Virginia will not engage in political activity on September 11th," Sara Craig, campaign manager in the battleground state, told supporters on Monday.
Vice President Joe Biden will speak at the Flight 93 memorial in Shanksville, Pennsylvania - another battleground state - where a hijacked United Airlines passenger flight crashed.
While the economy remains the foremost issue among voters, the Obama campaign has been increasingly on the offensive when it comes to foreign affairs.
At the Democratic convention last week, Obama's supporters spoke proudly of bin Laden's killing in 2011, highlighting it as a signature achievement for the president.
When accepting the Democratic presidential nomination, Obama attacked Romney as a foreign policy novice and pointed to the planned end of the war in Afghanistan and the rebuilding of One World Trade Center in downtown Manhattan as signs of American resurgence during his first term.
Romney has praised the U.S. killing of bin Laden, but said Obama's policies toward Iran had made the country less safe during his time in office.