WASHINGTON (AFP) - At 80, Mr John McCain has begun his sixth term in the US Senate by proving to be a thorn in Mr Donald Trump's side even before the president-elect takes the oath of office.
The maverick lawmaker is a long-time national security hawk and he has positioned himself as less an adversary of Mr Trump, 70, than an enemy of President Vladimir Putin.
But that posture has put him at odds with the incoming administration and Mr Trump's coziness with the Kremlin, as Mr McCain blasts Mr Putin as "a thug and a murderer" and calls for stricter sanctions on Moscow.
Mr Trump, who takes power on Jan 20, has repeatedly cast doubt on Russia's role in cyber attacks against the United States, which intelligence chiefs say was conducted in order to help Mr Trump win the election.
Mr McCain by contrast minces no words, calling Russia's alleged hacking "an act of war".
During the bitter 2016 presidential race, Mr McCain was criticised by some Republican voters for refusing to formally support the Republican nominee.
In Washington, he offers a national security reality check to Mr Trump, who is often accused of being in denial about Russia.
On Thursday (Jan 5), Mr McCain presided over a closely watched Armed Services Committee hearing in which senators grilled intelligence chiefs about Russia's role in hacking the Democratic National Committee.
Several of Mr McCain's staunch political adversaries in Congress now look to him as a bulwark against Mr Trump's perceived naivete about Russia.
"John's in a strong position," number two Senate Democrat Dick Durbin said. "Many of the statements that have been made during the course of the transition do not reflect the reality of America's national security, and I think John has brought that point home when it comes to our relations with Russia and the value of our intelligence agencies."
Republican Senate Foreign Relations Committee chairman Bob Corker hailed him as "an important voice".
The political novice and the warrior-senator are polar opposites.
Mr Trump, a billionaire businessman, never served in the military, and has struck a condescending tone when discussing America's generals or Gold Star families.
Mr McCain is a US Navy veteran, a pilot who was shot down over North Vietnam in 1967, beaten by an angry mob and bayoneted.
He spent five-and-a-half years as a prisoner of war, and his treatment was so brutal he still can not raise his arms high enough to comb his hair.
And yet Mr Trump belittled Mr McCain's experience when he was asked in 2015 whether he considered him a hero.
"I like people who were not captured," Mr Trump said.
The disdain appears to be mutual. Mr McCain bristles when asked about the incoming president.
"I do not want to be rude to anyone, but I do not want to be asked about Donald Trump," he snapped at reporters outside the Senate chamber.
Instead, he focuses on ways to combat Russian aggression. He has poked the bear on several occasions, including 2014, when he described the former communist power as "a gas station masquerading as a country".
He frequently visits Ukraine, including in 2013, when he joined protesters in Kiev's Maidan Square, and as recently as last month on a congressional delegation.
When Washington sanctioned Russia for its intervention in the Crimea, Moscow replied by publishing its own list of sanctioned US officials, including Mr McCain.
Now he wants to tighten the screws, working with other lawmakers to introduce new sanctions on Russia for its role in hacking US democratic institutions.
Elected in the House of Representatives in 1982 and serving since 1987 in the Senate, Mr McCain commands respect from colleagues, although they do not always approve of his independent streak.
He opposed isolationists in his party in 2013 when he worked with President Barack Obama in the hopes of launching military strikes against the Syrian regime of Bashar al-Assad.
At the time, in an interview, he scoffed at suggestions he underwent a metamorphosis from cynical lawmaker fighting Mr Obama at every turn to a remade Washington power player.
Mr McCain's rejection of that narrative "probably won't change the convenient storyline that the 'angry, bitter old man' that used to be the maverick is now back again", he said.
"But I think it's bullsh*t."
His national security battles are putting him in the spotlight once again.
With Mr Trump seeking detente with Washington's Cold War foe, time will tell whether Republican leaders in Congress fall in line, or embrace Mr McCain's confrontational strategy.