WASHINGTON — Sen. John McCain is waging another national campaign – this time, to define his legacy.

After two unsuccessful presidential bids, the 78-year-old former Navy pilot and Vietnam prisoner of war has rebounded as the chairman of the powerful Senate Armed Services Committee. The post gives the Arizona Republican a significant say on national security – and a chance to ensure that his loss to Barack Obama in the 2008 White House race isn’t the final word in the colorful McCain chronicles.

McCain wants to prod the Obama administration, which he derides as feckless, to adopt a tougher policy against worldwide threats. He wants budget and spending changes at the Pentagon.

A defense hawk in a party with a growing number of noninterventionists – he once dismissed a few as “wacko birds” – McCain wants to help educate new senators. McCain is calling foreign policy luminaries to share their world views with the committee, beginning this week with former national security advisers Zbigniew Brzezinski and Brent Scowcroft. His abiding friendships across Washington’s political tribes are points of pride.

But ask McCain what he wants people to think of when they will recall his chairmanship.

Then ask how he wants to be remembered generally.

The answers are nearly identical.

“To be able to play a significant role in defeating the forces of radical Islam that want to destroy America,” he says to the first.

To the second: “That I made a major contribution to the defense of the nation.”

It’s legacy time for McCain, and he clearly wants the chairmanship to help define it, before voters in 2016 get the chance again to decide control of the Senate. The senator turns 80 that year, and all signs point to McCain running for a sixth Senate term.

Two years is a short window for a lot of work, but the leadership role gives McCain new power to push his agenda. He still has the energy that has helped him survive a hard-to-makeup biography: three plane crashes, an aircraft carrier fire, five torturous years in captivity in Vietnam, nearly three decades in the Senate – and too many donut-fueled hours with reporters aboard the “Straight Talk Express” presidential campaign bus in 2000 and 2008.

“You will see, probably, the busiest Senate Armed Services Committee that you’ve ever seen,” McCain said in a recent interview.

Some members of the administration are eager to reset the sometimes-tense relations with the blunt-spoken, dry-humored senator. After all, knotty international issues, such as the prospect of using force against Islamic State militants, fall within the purview of McCain’s committee.

“John is quixotic,” Vice President Joe Biden said in a recent telephone interview.

The two are genuine friends from serving two decades together in the Senate, though it’s clear that Biden hasn’t forgotten “some interesting things he’s said about me, publicly” – such as McCain’s suggestion in 2012 that Obama drop Biden as his running mate.

“I know he loves me. And I care about him, I really do,” a chuckling Biden said of McCain. “I think John’s legacy is that he never quits.”

McCain has a similar relationship with Secretary of State John Kerry, also a former Senate colleague and, like Biden, a onetime presidential hopeful.

Last April, during a hearing, McCain ripped into Kerry for “talking strongly and carrying a very small stick – in fact, a twig” on foreign policy. Kerry rejected what he said was McCain’s “premature judgment about the failure of everything.”

For all his partisan bluster and fierce conviction, McCain has a record of dealmaking so established that it inflames some on the political right, Kerry said. The pair, both decorated Vietnam veterans, played key roles when President Bill Clinton normalized relations with Vietnam in 1995. More recently, McCain’s been a member of bipartisan Senate groups that worked out deals on judges and immigration, though an immigration overhaul stalled in the House.

“He’s a guy you want fighting with you if you can find a way” to compromise, Kerry said in a statement to The Associated Press. “This moment is a big one for him, and he has the capacity to really make a deep impact.”

McCain is a frequent target of the right, who complain about his work with Democrats, from the late Massachusetts Sen. Ted Kennedy to Democrat-turned-independent Joe Lieberman of Connecticut. The latter might have ended up as McCain’s running mate in 2008. Instead, McCain went with Sarah Palin.

On the Pentagon, McCain is looking to a longtime role model, Arizona Sen. Barry Goldwater, the GOP’s presidential nominee in 1964 who lost that race, then returned to the Senate to serve as chairman of the Armed Services Committee.

McCain said the committee will tackle Pentagon restructuring as a sequel of sorts to the 1986 Goldwater-Nichols Department of Defense Reorganization Act, which streamlined the military chain of command.

McCain wants to end the automatic spending cuts that affected the military. Early next month, he expects the committee to confirm Ashton Carter as Obama’s new secretary of defense. He would like the outgoing secretary, former GOP Sen. Chuck Hagel of Nebraska, once a close friend, to testify when the committee considers the Pentagon budget.

Witnesses, regardless of party, should not expect a comfortable experience, said someone who knows.

“He would go after me just as much as anybody else,” recalled ex-Defense Secretary William Cohen, a former Republican senator from Maine who served under Clinton.

“He can embarrass you,” Cohen said. “But he’s got a really good heart in terms of his sense of fairness.”