McCain, Obama's rival for the White House in 2008, has become one of the president's staunchest Republican allies in recent weeks as the Arizona senator butts heads with the obstructionist Tea Party wing of his party.
Has Maverick McCain returned?
As he's negotiated deals on a host of issues with Democrats and the Obama administration, reportedly talking to senior White House staffers several times a week, the 76-year-old McCain has chastised fellow Republican lawmakers like Ted Cruz and Rand Paul — famously branding them "wacko birds" at one point.
The Arizona senator even mocked Paul's rumoured presidential aspirations this week, saying sarcastically it would be a "tough choice" for voters to decide between the Tea Party darling and Hillary Clinton in 2016.
In a piece entitled "Can Anyone Stop John McCain?" the right-wing National Review branded McCain "Obama's secret weapon." The story said conservative Republicans fear he's posing a "lethal threat" to the party's hopes to win a showdown against the president over the federal budget in Congress this fall.
Cruz has suggested McCain and his moderate ilk in the Republican party are "scared" of being beaten up politically if they back the Tea Party's hardline stances on everything from immigration reform to so-called Obamacare.
Conservative Republicans are deadset against a path to citizenship for the country's 11 million illegal immigrants without significantly tougher border security measures, and they're threatening to shut down the government in September if Congress doesn't vote to defund Obamacare, the president's sweeping health-care reform law that was upheld by the U.S. Supreme Court a year ago.
McCain, a former Vietnam prisoner of war, scoffed at Cruz's assertion in an interview with ABC News, broadcast Friday.
"We need to be careful in the way we treat each other," McCain said.
"I have been as ferocious a fighter and I think as partisan and strong as anybody, but I really try hard not to get personal. Debate on the issue as hard as you can, but don't say that your opponents, people who disagree with you, are scared. It's been a long time since I've been scared."
He also issued a warning for the freshmen Republican lawmakers in Congress who are threatening the government shutdown, saying they might need some "corporate memory."
"The people who are pushing this Obamacare versus government shutdown — none of them that I know were here the last time we saw that movie," he said.
And McCain reiterated his frequent admonishment since the November 2012 election: Republicans will not win back the White House if Congress fails to pass significant immigration reform over the next three years.
"It won't matter who our nominee is, because of the polarization of the Hispanic vote," he said. "That's not why I'm for immigration reform, but it certainly is one of the consequences of a failure."
McCain has been sitting down with several Senate Republicans in recent weeks, and the White House, in an attempt to come up with a deal that would prevent, once again, another government shutdown or a loan default. The government runs out of money on Oct. 1, and Congress must vote to approve more funding before then to avoid a shutdown.
Last month, McCain negotiated another deal that allowed Obama to fill seven top administration officials despite Republican vows to stop him.
He was also one of the bipartisan "Group of Eight" senators who produced a comprehensive bill to overhaul the U.S. immigration system — legislation that's now in its death throes in the more conservative House of Representatives.
McCain's newfound friendship with Democratic Sen. Chuck Schumer following their work together on the immigration bill — in addition to the Arizona lawmaker's longtime friendship with Denis McDonough, Obama's new chief of staff — is thought to be behind the supposed return of the maverick, now considered one of the shrewdest and most effective negotiators in Washington.
McCain and Schumer were positively giggly in a recent interview with Politico.com, with McCain describing how they've struck up a friendship "in a strange, unusual fashion."
Amid the affectionate barbs between the two, however, McCain suggested his new role as a top bipartisan deal-maker has more to do with Obama than a sudden change of mood.
"I still relish the role of a grumpy old man," he joked, but insisted he's always been guided by his principles and is neither a maverick nor a crank.
"I have always been result-oriented, and now I have more opportunities than maybe we had in the first two years of the Obama administration .... Every president in their second term is looking at their legacy, and I know that this president is looking at his legacy, and frankly, on the issue of the immigration reform, he did literally everything that we think was productive in this whole scenario."
McCain is hardly a total Obama fanboy, particularly on foreign policy.
He remains one of the administration's harshest critics on Benghazi, and has long decried Obama's failure to intervene in the bloody Syrian civil war. McCain has vowed to continue to "pound" the president on both issues, despite their unity on domestic matters.
His return to the centre of the political spectrum comes three years after he careened to the right when facing a primary challenge during the 2010 mid-term elections from a Tea Party darling. McCain, a longtime champion for immigration reform, even aired a TV ad that urged Obama to "build the danged fence" along the Arizona-Mexico border.
McCain isn't facing re-election until 2016 — and is therefore expected to remain a thorn in the side of his far-right Republican colleagues for the next three years.