WASHINGTON - The joint appearance at ground zero this morning by Sens. John McCain and Barack Obama will not only commemorate the seventh anniversary of the Sept. 11 attacks but also will mark a rare moment in the campaign when both candidates focus on terrorism, an issue that has lost salience for voters as the attacks recede in the public memory.

Once the key concern that propelled Republicans to big electoral victories in 2002 and 2004, terrorism has often seemed the forgotten issue of 2008. Both candidates touched on the subject only briefly in their convention speeches and are instead emphasizing the economy, change for Washington and other issues on the stump.

The shift reflects a nod to the changing attention of the electorate. At this time in 2002 and 2004, about a quarter of all Americans polled by Gallup called terrorism or national security the country's top problem. That dropped to 16 percent in 2006, and now 4 percent of those polled deem those issues the most important challenge facing the United States.

"The whole issue has not gotten anywhere near the attention most people would have predicted four years ago," said Paul Pillar, a leading authority on terrorism and a retired CIA analyst. "It is kind of striking that this set of issues that became such a huge national preoccupation in the years after 9/11 has faded so much."

Still, both Obama and McCain have tried to use the terrorism issue to emphasize larger themes about their candidacies and to question the judgment of their adversary. The McCain camp has suggested that Obama would adopt a law enforcement model it says was favored by the Clinton administration in the 1990s.Obama's advisers say McCain is too fixated on the use of military power.

Conflict in Iraq is still a touchstone

The war in Iraq remains a touchstone: McCain regards the conflict as central to a war with Islamic terrorists, whereas Obama has described Iraq as a distraction from what should have been the main focus on Afghanistan.

"Obama persists in the fiction that Iraq was never a central front in the war on terror," said Randy Scheunemann, McCain's top foreign policy adviser. "Had we left Iraq in the time and manner Senator Obama advocated, Al-Qaida would have achieved a victory that dwarfed what they believe they achieved in Somalia -- it would have been devastating for our interests." Scheunemann was referring to the withdrawal of U.S. forces from Somalia during the Clinton administration, a move that Al-Qaida has used for propaganda.

Obama advisers emphasize his call in the summer of 2007 for more troops and resources in Afghanistan, where the United States and its allies have been battling a resurgent Taliban militia in recent months. "He called for sending two brigades of U.S. forces to Afghanistan 13 months before the chairman of the Joint Chiefs of Staff said the same thing," said Richard Clarke, the onetime Bush and Clinton adviser who has been chairing an advisory panel on counter-terrorism for the Obama campaign.

Despite the rhetorical differences, the candidates shared important similarities. Both McCain and Obama have called for the closure of the Guantanamo Bay, Cuba, detention facility for suspected terrorists and more humane interrogation procedures for suspects.

They also both voted in the Senate for the wireless surveillance program promoted by the Bush administration.