In the aftermath of the Arizona shootings, President Obama ordered that American flags be flown at half-staff and called for a national moment of silence, which he and first lady Michelle Obama observed on the South Lawn of the White House at 11 a.m. Monday.

Silence was the easy part. Now the question becomes, when Obama next addresses the nation, what he will say.

White House officials are weighing their options. Obama will travel to Tucson on Wednesday to attend and address a memorial service for the victims of Saturday's shootings.

The service is set for 8 p.m. Eastern time at the University of Arizona's basketball arena, the school said. It will include a Native American blessing, a moment of silence, a poetry reading and the presentation of a chain featuring messages from members of the public, the school announced.

Obama is likely to deliver a speech about tolerance, a theme that could also be featured in his State of the Union address on Jan. 25.

However, with liberals and conservatives assuming their assigned battle stations over whether gun laws and partisan rhetoric are to blame, the White House is undecided about the exact message the president will send.

It is not clear whether ideology motivated the alleged shooter, Jared Loughner, and Obama's advisers may conclude it unwise for the president to lecture the nation on mutual respect - which could leave him open to criticism that he is using the tragedy for political gain.

Obama canceled plans to travel to Schenectady, N.Y., on Tuesday to give an economic speech. Few events were on his calendar this week: He met with French President Nicolas Sarkozy at the White House on Monday, and he is set to attend the funeral of U.S. ambassador Richard C. Holbrooke in Washington on Friday.

The president will settle in this week with part of his newly formed West Wing team, as Chief of Staff William Daley and senior adviser David Plouffe take on their new roles.

Daley happened to be at the White House on Saturday when news of the shootings broke. He was whisked into his first Situation Room meeting with the president before his official duties had even begun. His next task may be helping shape the president's Arizona message.

The words that commanders in chief choose in times of national distress can define their presidencies. Historians cite Abraham Lincoln at Gettysburg in 1863; Ronald Reagan after the 1986 Challenger disaster; Bill Clinton after the Oklahoma City bombing in 1995; and George W. Bush's National Cathedral speech in the aftermath of the Sept. 11, 2001, attacks as examples of presidential speeches that transcended politics and united the country, however briefly.

Kathleen Hall Jamieson, director of the Annenberg School for Communication at the University of Pennsylvania, calls such addresses "national eulogies" and said Obama has an opportunity to deliver one in the days ahead.

"The challenge for a president is trying to make meaning out of something that's not easy to understand - something that looks to be a hugely disrupting act," Jamieson said. "A president is trying to provide a framework for how we are going to understand it."

But there are also reasons for a president to pause before weighing in at such a moment. Investigators are still gathering evidence about the shooter's motives. Before discussing the attack in the context of inciting political rhetoric, the proliferation of guns, or a lack of care for the mentally ill, Jamieson said, Obama should collect as much information as possible about what happened and why.

Reagan delivered his moving remarks about the Challenger explosion only hours after it occurred, largely because what happened was clear.

Clinton, by contrast, waited four days before traveling to Oklahoma City to deliver remarks at a prayer service for the victims. Until then, he participated in other commemorative events.

kornbluta@washpost.com wilsons@washpost.com