"Petty and classless," said Rep. Brendan Boyle (D-PA). "In other words, Trumpian."
The feud got going in 2015, as then-candidate Donald Trump was just beginning his campaign for the White House; at an event, Mr. Trump said he didn't consider McCain a war hero, because McCain had been captured and held prisoner in North Vietnam.
That drew rebukes from both sides, but it thrilled many of the President's most ardent supporters, who considered McCain a GOP turncoat.
The bad blood continued as McCain refused to support Mr. Trump in the 2016 elections, during the transition, and after the President took the oath of office.
"I will not talk about Donald Trump," McCain told reporters at one point after the election, though he went on for months to do exactly that in the hallways of the Capitol, unable to get away from the President's actions on a host of policy fronts.
One of the biggest was on the issue of health care, as Republicans tried to get rid of the Obama health law. But after weeks of arm-twisting, the Senate waited for McCain to arrive on the floor after midnight to cast the deciding vote.
To gasps on the Senate floor, McCain gave a big "thumbs down," dooming the last-ditch GOP health care bill backed by the President, further earning the ire of Mr. Trump.
Even after going back home for treatment of his cancer, McCain continued to sternly criticize Mr. Trump, especially over foreign policy matters and the President's dealings with Russian leader Vladimir Putin.
"President Trump must be willing to confront Putin from a position of strength," McCain said in one statement, demanding tougher sanctions on Moscow, and raising questions about whether Mr. Trump should meet with the Russian leader in Helsinki.
That summit meeting did occur - and the news conference by the two leaders which followed brought forth a storm of criticism, led by McCain.
At campaign rallies in recent months, the President has repeatedly expressed his frustration with McCain's health care vote - as even without saying McCain's name - the mere reference would draw a negative reaction from the crowd.
President Trump also did not mention McCain's name several weeks ago, when signing a major defense policy bill into law - which bore McCain's name, as an honor to the Arizona Senator.
McCain's family said that eulogies would be given at Saturday's National Cathedral service by former Presidents George W. Bush and Barack Obama - as it seemed possible that President Trump would not join in any of the Washington ceremonies to honor McCain, who was first elected to the House in 1982, then to the Senate in 1986.
Reportedly, Vice President Mike Pence will attend the Saturday service instead of President Trump.
"We honor his lifetime of service to this nation in our military and in public life," the Vice President tweeted soon after McCain's death was announced.
"God Bless John McCain," Pence added.