McCain has long been the GOP’s most respected voice on national security matters, relentlessly challenging the Obama administration on everything from defense spending cuts to the Sept. 11 attack at the U.S. consulate in Benghazi, Libya.

But the former Navy pilot and presidential contender has hit his six-year term limit as the ranking member of the Armed Services Committee and is not expected to seek a waiver from GOP rules.

One option for McCain would be to take the top Republican slot on the Homeland Security and Governmental Affairs Subcommittee on Investigations, a perch from which he could launch probes into wasteful defense spending and other government spending.

McCain has “had an active interest in investigating fraud, waste and abuse issues,” said one defense source who tracks the Senate Armed Services Committee. “It’s not beyond the pale of possibilities.”

Oklahoma Sen. Tom Coburn, the investigation subcommittee’s current ranking member, is expected to succeed Maine Sen. Susan Collins as the top Republican on the full committee. Like McCain, Collins is facing a term limit on the post.

To be sure, the investigations subcommittee post does not have the same cachet as being Senate Armed Services ranking member. But McCain would have at his disposal a team of investigators to probe inefficiencies and fraud across the government — a hallmark of his time on Senate Armed Services and as chairman of the Indian Affairs Committee.

The subcommittee post would also allow him to continue working closely with Armed Services Chairman Carl Levin of Michigan, who chairs the investigations panel.

The two men have already teamed up on extensive investigations, including a yearlong probe into the prevalence of counterfeit electronic parts from China used on U.S. military planes, helicopters and other equipment.

During their time at the helm of the Armed Services Committee, McCain and Levin have demonstrated an ability to work together, getting a defense authorization bill through the committee each year despite partisan differences and White House opposition to some provisions contained in the sprawling policy measure.

McCain is expected to remain a member of the Senate Armed Services and could take a leadership role on a subcommittee.

The Airland panel, which oversees Army and Air Force programs and which McCain previously chaired, will soon have an open spot now that Sen. Scott P. Brown of Massachusetts lost re-election. Another possibility would be the Seapower Subcommittee, from which McCain could continue to oversee big-ticket Defense Department programs such as the F-35 Joint Strike Fighter and Navy ships.