“I don’t understand how anybody can be held responsible for somebody who is completely mentally unstable like this,” an adviser to Ms. Palin, Rebecca Mansour, said in an interview with a conservative radio host, Tammy Bruce. Responding to accusatory messages on the Web, Ms. Mansour added: “People actually accuse Governor Palin of this. It’s appalling  appalling. I can’t actually express how disgusting that is.”

Ms. Mansour said that the cross hairs, in fact, were not meant to be an allusion to guns, and agreed with her interviewer’s reference to them as “surveyors symbols.” Aides to Ms. Palin did not respond to interview requests on Sunday.

The Arizona rampage upended the opening agenda for the 112th Congress, particularly efforts by the new Republican majority to repeal the new health care law.

“This inhuman act should not, and will not, deter us from our calling to represent our constituents and to fulfill our oaths of office,” said Mr. Boehner, who presided over a unity conference call with hundreds of Republican and Democratic lawmakers on Sunday. “No act, no matter how heinous, must be allowed to stop us from our duties.”

The president ordered that flags be flown at half-staff and called for a national moment of silence at 11 a.m. Monday, which aides said he would observe from the White House South Lawn. He canceled an economic trip to New York on Tuesday.