A 9/11 commission member, two Senators, the head of FedEx or a former top State Department official could each become Defense secretary if Sen. John McCain (R-Ariz.) wins the presidency — but increasingly, defense experts and political observers believe McCain might not make a pick at all, instead keeping current Pentagon chief Robert Gates in place.

Congressional aides say that keeping Gates would be a popular choice on Capitol Hill. Both Democratic and Republican aides said Gates has earned the respect of lawmakers and has shown a willingness to work in a bipartisan manner.

“McCain carrying over a guy that the Democratic Congress gets along with could make life a lot easier for him,” said James Carafano, a defense expert with the Heritage Foundation.

McCain has not directly commented on whether he’d like Gates to stay on. But as ranking member of the Senate Armed Services Committee, McCain has developed a far better working relationship with Gates than with his predecessor, Donald Rumsfeld, someone the Senator labeled “one of the worst Defense secretaries in history.”

McCain and Gates are generally seen as being on the same page about the Iraq War. The Arizona Senator had long ago called for more troops in Iraq — a “surge” plan that Gates has carried out, with largely positive results, since taking the post in 2006.

Gates has not completely ruled out staying at the Pentagon, but he has told reporters he does expect to leave his post when Bush leaves office in January.

Pentagon observers note that Gates has been unusually busy for a Cabinet official who’s expecting to leave office in only five months. He issued a new national security strategy this summer, and he ousted the Air Force’s top military and civilian officials in July due in part to their poor handling of a tanker aircraft contract award and problems in the Air Force handling of nuclear weapons.

McCain has long been a critic of the Air Force tanker contract and pushed for an investigation several years ago that sent a top Air Force and Boeing official to jail for negotiating a sweetheart deal to build the planes. In a statement after the firings, McCain backed the move, calling Gates’ decision “decisive” and one that came after “careful consideration.”

However, some campaign aides and defense experts expect that McCain would be eager to bring in his own Pentagon chief and foreign policy advisers.

Those frequently cited as possible Defense secretaries are Sens. Joe Lieberman (ID-Conn.) and Lindsey Graham (R-S.C.), who both have campaigned frequently for McCain; John Lehman, a former Navy secretary and member of the 9/11 commission; Richard Armitage, who served as deputy secretary of State from 2001 to 2005; and Fred Smith, the founder of FedEx.

Among Senators, Lieberman is the most frequently mentioned, possibly for the top slots at Defense, State or Homeland Security. Lieberman is an outspoken supporter of the Iraq War, is close to McCain and has spent nearly two decades on the Armed Services Committee developing military expertise.

Moreover, the Independent Democrat could lose his chairmanship of the Senate Homeland Security and Governmental Affairs Committee if Democrats gain seats in the Senate this fall. Being cut loose from that perch would give him little incentive to remain in the Senate.

Meanwhile, Graham, an Air Force Reserve lawyer who along with McCain has been a harsh critic of the Bush administration’s treatment of military detainees, is also mentioned as a Pentagon possibility, but he has said several times he does not want the job. Graham would also have to give up a safe Senate seat that he’s expected to win for a second term this fall.

The South Carolina lawmaker declined to offer names of possible Pentagon chiefs for McCain, but he said that the presumptive nominee, if elected, will have no problem breaking from some of the more controversial Bush war policies.

“John is the change himself. He has been arguing with the Bush administration since the fall of Baghdad,” Graham said. “He’s got the vision. He just needs the right people around him to help implement it.”

If McCain does not choose a Senator, many observers say he may tap Lehman, who like the GOP nominee is considered something of a maverick.

Lehman rankled some Pentagon brass in the 1980s by pushing for a 600-ship Navy and later caught some Republicans by surprise as a member the 9/11 commission by criticizing both President Bush and former President Bill Clinton for underestimating Osama bin Laden. A Navy and Air Force veteran, Lehman has been personally close to McCain for more than three decades and currently runs a private equity firm.

Armitage has been a key foreign policy adviser to the McCain campaign and shares some of his more moderate views on diplomacy. Like McCain, Armitage is a Naval Academy graduate and has served in a variety of Pentagon and State Department positions under the past three Republican administrations. He’s also been touted by some as a possible secretary of State.

Several political observers, however, doubt that Armitage would be nominated for such a prominent post because he’d face tough confirmation questions about his role in the Valerie Plame affair. He has admitted — after a years-long investigation of top Bush White House advisers — to leaking to some journalists the sensitive information that Plame worked for the CIA.

Meanwhile, Smith, a former Marine who served two tours in Vietnam before founding FedEx, has been mentioned for both economic and defense posts. But political observers say Smith, a former fraternity brother of President Bush, is not personally close to McCain, and many believe that he’d be reluctant to name an outsider as Defense chief with wars ongoing.

Sen. John Thune (R-S.D.), who sits on the Armed Services Committee, said that lawmakers are likely to defer to McCain in choosing his Defense secretary. “I’d hate to offer any names because John has so much military experience himself,” he said.