Washington (AFP) - Ailing US Senator John McCain in his new memoir decried the "scarcity of humility" in today's politics and criticized Donald Trump's aggressive bluster, warning that the president is concerned more with faking toughness than promoting American values.

"This is my last term," the 81-year-old McCain, who is home in Arizona battling brain cancer, wrote in his book "The Restless Wave," according to excerpts published by Apple News. "I can speak my mind without fearing the consequences much."

McCain, a former prisoner of war in Vietnam, was first elected to Congress in 1982 and became a titan of US politics.

While he anticipates advances in cancer treatment, "I don't know how much longer I'll be here. Maybe I'll have another five years," he wrote.

"Maybe I'll be gone before you read this."

Serving his sixth term in the US Senate, McCain expressed eagerness to help heal a political divide that has only deepened in recent years, writing "you're damn right, I'm a champion of compromise."

He also sounded a warning about America's polarization and the "ideological ghettos" in which political zealots have secluded themselves.

"Increasingly, we have our own facts to reinforce our convictions and any empirical evidence that disputes them is branded as 'fake,'" he wrote in his book, which is set for a May 22 release.

"There is a scarcity of humility in politics these days," he added. "If it vanishes entirely, we will tear our society apart."

Despite being in the same party as the president, McCain has been a forceful Trump critic over the last two years, both in the run-up to the 2016 election and its aftermath.

McCain appeared not to be holding much back about Trump in his memoirs.

"He has declined to distinguish the actions of our government from the crimes of despotic ones," McCain wrote of the president.

"The appearance of toughness, or a reality show facsimile of toughness, seems to matter more than any of our values."

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McCain, who was the 2008 Republican presidential nominee and lost that year to Barack Obama, has served alongside six presidents.

"I've disagreed, sometimes too heatedly, with all of them," he said.

But such disagreements, he stressed, should never diminish the respect that lawmakers and everyday Americans have for one another, even in times of turmoil.

McCain has not been to Washington since December, five months after being diagnosed with glioblastoma, an aggressive form of brain cancer.

On Monday, McCain's wife Cindy tweeted that they "enjoyed a wonderful visit" at the weekend from former vice president Joe Biden, McCain's longtime Democratic Senate colleague.

McCain wrote that while he will "hate to leave" this world, "I don't have a complaint. Not one. It's been quite a ride. I've known great passions, seen amazing wonders, fought in a war, and helped make a peace."