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WASHINGTON — The Senate was out of session Monday, but John McCain was on Capitol Hill, joking, as he often does, about "trying to do the Lord's work here in the city of Satan."

On this particular day, the Lord's work involved a bit of bowel humor. "There was a poll recently on the favorability of everything in American life," he told a gathering in the Russell Senate Office Building, upstairs from his office. "The favorability numbers of Congress ranked just below a colonoscopy. So we are trying to raise it at least above that."

The Arizona Republican is certainly doing his part to raise his colleagues' image above that of the intestinal probe. On Sunday, he appeared on CNN, praising President Obama's speech on the George Zimmerman case and proposing a review of the "stand your ground" laws that came to attention because of the killing of Trayvon Martin.

Days earlier, McCain had brokered a deal averting a procedural meltdown in the Senate over the filibuster. Before that, he was a key figure in drafting the bipartisan immigration bill that cleared the Senate. Also this year, he has called some of his hard-line Republican colleagues "wacko birds" and has criticized them for blocking a budget resolution.

All of this is to say the Mac is Back. Again.

It has been a long wait. The maverick of 2000 and the thorn in George W. Bush's side during the years that followed became a very different figure as he captured the 2008 Republican nomination, gave us Sarah Palin, fought against health care reform and staved off a tea party challenger in 2010. He walked away from earlier positions on tax cuts, campaign finance reform, immigration, climate change, gays in the military and other issues. When he continued that course even after the 2010 election, I figured he had been lost to the right wing.

For 20 minutes Monday, I strolled with McCain in the Russell building and sat in his office, getting his perspective on his retirement as maverick and subsequent reactivation. He acknowledged that he changed his emphasis on immigration in part "to be in tune with the people of Arizona," and he said he thought it his duty as the party's former nominee to wage "a bitter fight" against Obamacare.

But he argued that the bigger change was in the circumstances. During the early Bush years, when Republicans were firmly in control of government, he resisted his own party and voted against Bush's early tax cuts. After Democrats took the reins, he turned his antagonism against them. Now power is divided, and McCain sees much of the problem coming from fellow Republicans.

"There's a view," he said, "that a reason for coming here is not to get something done but to prevent anything from getting done." He cited the standoff on a budget resolution: Republicans beat up on Senate Democrats for years for failing to pass a budget, and now that the Senate has finally passed one, Republicans are objecting to the Senate naming conferees to negotiate a budget with the House.

He's also got a problem with the roughly 15 isolationists in the Senate GOP caucus, a few of whom earned his famous "wacko birds" label. McCain claims he borrowed the phrase from the Wall Street Journal — "not that I disagreed with it."

Many on the left won't forgive McCain his trespasses in 2008 and 2010, and many on the right won't forgive what he did before and since. But McCain has arguably turned himself into the most important legislator in a generation, at the center of the debate on war, terrorism, spending, corruption, health care and just about everything else. Last week, McCain met for two hours with Obama and Vice President Biden on security matters; Monday morning, he had a tete-a-tete with Secretary of State John Kerry.

When I caught up with him Monday afternoon, he was pitching legislation to replace the dollar bill with a coin; next is legislation allowing people to order cable channels a la carte.

McCain laughed frequently during our meeting, and seemed to relish his return to the indispensable-man role. "What else?" he kept asking me, after I had exhausted my list of questions. He claimed not to be discouraged by the colonoscopy-level image of Congress. "My strength and weakness is I enjoy being in the arena," he said, quoting his hero, Teddy Roosevelt. "I like the fight."

That's good news. The important thing is not where McCain has been but that he's back. He's needed more than ever.

Follow Dana Milbank on Twitter, @Milbank.