One of the president's chief critics could soon lead the Senate's main military committee.

With Republicans gaining the majority in the upper chamber in Tuesday's midterm elections, Sen. John McCain is widely expected to become the next Armed Services Committee chairman in January.

McCain, the Republican Party's presidential nominee in 2008, has decades of experience in foreign policy and defense issues in the Senate, where he was first elected in 1986. He also served in the Navy, and he spent more than five years as a prisoner of war after his plane was shot down during the Vietnam War.

As committee chairman, McCain would have an influential role in spearheading defense policy from Capitol Hill. That includes the Senate's version of the National Defense Authorization Act, an annual bill that outlines defense policy and tells the Pentagon what it can and can't spend money on. He'll also gain a megaphone to voice his frequent opposition to the Obama administration on military and national security issues.

On Tuesday, Pentagon spokesman Rear Adm. John Kirby downplayed any concerns over McCain gaining the top spot, but, at least publicly, the senator's relationship with the Pentagon has been rocky.

For example, during the past year, McCain put a hold on Bob Work's nomination to be deputy Defense secretary, called the administration "cowardly" for not providing arms to Ukraine's military, and frequently criticized the strategy to combat the Islamic State of Iraq and Syria.

And his blunt style isn't likely to change once he becomes chairman.

"With Senator McCain, what you see is what you get. It's part of his charm and persona, that he is a maverick—he is an outspoken maverick—that tells it like it is to anyone of any party," said Mackenzie Eaglen, a fellow at the Washington-based American Enterprise Institute think tank. "He wasn't best friends with the Defense Department when the last president was in office either."

Despite McCain's penchant for straight talk, he's also known for a willingness to work with his opponents and find a solution—for example, on immigration-reform legislation with the "Gang of Eight." And there are at least two areas where he and top Pentagon officials agree that something has to change: budget cuts under sequestration and how the Defense Department buys what it needs.

Without action from Congress, the budget caps would return in October 2015, the start of the 2016 fiscal year. Under Obama's five-year budget, the Pentagon projects that it will need more than $535 billion in fiscal 2016. But under the budget caps, the Pentagon is expected to receive less than $500 billion, leaving a roughly $35 billion budget gap.

McCain has been pressing for years to reverse or replace the cuts. And Eaglen called a budget deal—similar to the Ryan-Murphy agreement that eased budget cuts for fiscal 2014 and 2015—"a no brainer." But Eaglen also acknowledged that completely undoing the sequester for the Pentagon is "much easier said than done."

McCain is also expected to be outspoken on reforming the Pentagon's acquisitions process—or how the department buys the programs and technology it needs. He released a report that outlined potential improvements to the system last month with current chairman Sen. Carl Levin.

McCain will likely have an ally in Republican Rep. Mac Thornberry, the front-runner to replace Rep. Buck McKeon, as chair of the House Armed Services Committee, and Frank Kendall, the Pentagon's under secretary of Defense for acquisition, technology, and logistics. Kendall has been working on a reform effort, with the final draft expected to be released early next year.

"That's a really powerful trifecta of people that care about acquisition reform," Eaglen said.

But that also likely means McCain will be giving some tough talks to industry officials. He's been particularly critical of Lockheed Martin's F-35 aircraft, which has faced a series of delays and budget problems, as well the Navy's littoral combat ship. He called the small vessel, which was supposed to help add flexibility to the Navy's fleet, "over budget, behind schedule, [and] deficient" during a Senate hearing earlier this year.

But one industry official dismissed speculation that McCain will have a combative relationship with industry or the Pentagon.

"He expects industry to give him straight answers, and holds us accountable. I would hope that he would hold the Pentagon accountable," the official said, requesting anonymity so they could speak candidly. "One of the things I think needs to happen in the Congress in general is a greater amount of oversight versus just reviewing line-item details in the budget."

Technically, McCain hasn't been named to the committee's top spot yet. Chairmanships likely won't be announced until next month, but McCain has made no secret of the fact that he's long coveted the position. He told a Phoenix radio station earlier this year, "I would be so happy to be chairman of the Armed Services Committee."

And in the Senate, where chairmanships largely follow seniority, McCain has a lock on the job. He outranks Sen. Jim Inhofe, the committee's current ranking member. The Oklahoma Republican will reportedly head up the Senate Environment and Public Works Committee next year.

McCain, whose staff didn't respond to request for comment, previously served as the ranking member for six years. His party was in the minority during that period, and according to Senate Republican rules that govern committee selection, he can serve up to six years as chairman before being term limited.