WASHINGTON (Reuters) - If John McCain’s illness requires a long absence from the U.S. Senate, his colleagues will be deprived of a dealmaker and leading voice on national security, while America’s allies will lose one of the few Republican critics of President Donald Trump’s foreign policy.

FILE PHOTO - U.S. Senator John McCain (R-AZ) attends the Senate Armed Services Committee hearing on worldwide threats on Capitol Hill in Washington, U.S. on May 23, 2017. REUTERS/Yuri Gripas/File Photo

The 80-year-old lawmaker and 2008 Republican presidential nominee, who was elected to a sixth Senate term last November, has been diagnosed with an aggressive form of brain cancer. He is vowing a quick return to the Senate.

McCain, a former U.S. Navy pilot who spent 5-1/2 years as a prisoner of war in Vietnam, is outspoken on a range of issues, from defense spending to immigration to demanding a thorough investigation of alleged Russian meddling in the 2016 U.S. election.

Asked how the Senate was different without McCain, his close friend Senator Lindsey Graham said: “It’s quieter. John is a fighter and John’s into every cause no matter how hard it might be.”

A Russia hawk, McCain has expressed deep skepticism of Trump’s effort to improve ties with Moscow, and emerged, with Graham, as perhaps the most vocal Republican critic in Congress of the president’s foreign policy. He chairs the influential Senate Armed Services Committee.

McCain has traveled the globe on trips some analysts say are efforts to soothe the concerns of U.S. allies that fear Trump’s “America First” policy means a retreat from international engagement.

“I realize that some of President Trump’s actions and statements have unsettled America’s friends,” McCain said in a speech in Australia in May. “They have unsettled many Americans as well.”

McCain is also known for an independent streak and a willingness to work with Democrats. He has participated in almost every major bipartisan legislative effort in the Senate in recent years, such as the “Gang of Eight” immigration push in 2013.

In 2015, McCain, who was tortured while a POW, worked with Democratic Senator Dianne Feinstein to pass legislation banning torture. This week, he called for a bipartisan effort to overhaul the U.S. healthcare system.

DEFENSE SPENDING

While McCain has made clear his desire for more spending on defense, he has criticized what he sees as inefficiencies in U.S. weapons programs. Some analysts said defense contractors might have an easier time with another Armed Services chairman.

“McCain is McCain, and he has needled people and programs that he does not think are performing,” said Mark Cancian, of the Center for Strategic and International Studies International Security program.

McCain has criticized defense programs like the Littoral Combat Ship made by Lockheed Martin Co LMT.N and Australia's Austal Ltd ASB.AX. "LCS has been one thing that has been in his sights, and arguably there has been an effect," Cancian said.

To carry out Trump’s goal of repealing and replacing former Democratic President Barack Obama’s signature Affordable Care Act, popularly known as Obamacare, Senate Majority Leader Mitch McConnell needs the backing of at least 50 of the 100 U.S. senators. Republicans control the Senate by a narrow majority of 52 to 48.

As McCain weighs treatment options in his home state of Arizona, his absence makes it more difficult for McConnell to muster the support he needs to advance healthcare legislation expected to come to a vote next week.

“Obviously, I think more people are worried about his health than thinking about the math, but you know, you understand the math,” Republican Senator Bob Corker told reporters. “Obviously, it makes things difficult.”

Sarah Binder, a scholar at the Brookings Institution, said that with the slim Republican majority, the absence of any senator could be potentially “catastrophic” for the party’s effort to pass its agenda.

She said the problem was compounded by McCain’s absence.

“His colleagues perceive him as a ‘giant’ of the Senate, whose expertise and perspective on foreign and military affairs is arguably irreplaceable,” Binder said.