BOSTON — Trumpeting an endorsement from the chairman of the September 11 Commission, Senator McCain is seeking to position himself as the presidential candidate most qualified to protect America from potential terrorist attacks.

This line of campaigning puts the Arizona senator in direct conflict with Mayor Giuliani, who served as New York City mayor during and immediately after the attacks of September 11, 2001, and who has made his leadership during that period a cornerstone of his presidential bid.

Thomas Kean, who served as chairman of the commission set up to examine the circumstances around the attacks, and as governor of New Jersey between 1982 and 1990, refrained from criticizing Mr. Giuliani or the other Republican candidates yesterday. But his endorsement underscored his belief that Mr. McCain is the better man for America’s top job.

“Few others understand the dangers of a terrorist threat that continues to confront every single one of us every single day,” Mr. Kean said at Logan International Airport. “Senator McCain has been at the forefront of every effort taken in the aftermath of 9/11 to make the American people safe.”

Mr. Kean cited Mr. McCain’s role in helping to pass the Intelligence Reform and Terrorism Prevention Act of 2004 and, after prompting by reporters, referred to the senator’s efforts to improve communication between emergency response workers, an issue Mr. Giuliani’s opponents have sought to use against him.

Mr. McCain “supported recommendations to make first responders able to communicate with each other so you don’t have the kind of tragedy, the death of firemen, that happened on 9/11,” said Mr. Kean, who added that the endorsement is his first on the national political stage in two decades.

At times, Mr. Kean’s somewhat awkward refusal to cite other candidates by name prompted Mr. McCain to delve into the fray himself. After Mr. Kean offered a vague answer to a question about whether he believed it was “dangerous” if America elected a candidate without national security experience, Mr. McCain said, “Of course, I think it’s important. I have 20 years of experience on national security issues. … I need no on-the-job training.”

The event also represented a change in emphasis for Mr. McCain, who has referred in a number of speeches and written commentary to President Theodore Roosevelt and President Reagan. Mr. Kean compared Mr. McCain favorably with President Eisenhower, who he said “brought an honorable end to what appeared to be endless stalemate in Korea.” Queried about the comparison to a new president, Mr. McCain responded at some length. He began by citing a new history of the Korean War, “The Coldest Winter,” published posthumously by David Halberstam.

“That book describes the political climate in America today. … It really is interesting, the tension within American politics and the tension within the Republican Party, the Taft wing of the party versus the Eisenhower wing of the party,” Mr. McCain said. “We were blessed, quite frankly, by the presidency of President Eisenhower, and historians are judging him more and more favorably because he did keep us out of war. In fact, another war he kept us out of at the time was Indochina. He just said, ‘No dice.'”

With yesterday’s announcement, the Kean family formally divided on presidential endorsements. Mr. Kean’s son, state Senator Thomas Kean Jr. of New Jersey, has endorsed Mr. Giuliani.

“Mayor Giuliani is a tested leader with a commitment to national security and fiscal responsibility. Senator McCain is an honorable man, but I saw firsthand what Mayor Giuliani’s leadership meant to New York City, our region and the country as a whole,” the younger Mr. Kean said in a statement circulated by the Giuliani campaign.