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* Obama's 2008 opponent may be key to White House success
* Draws praise from Democrats, fire from right-wingers
* 76-year-old maverick "back in his groove"
By Thomas Ferraro
WASHINGTON, Aug 2 (Reuters) - Republican U.S. Senator John McCain accidentally walked in on President Barack Obama this week when he was addressing a Democrats-only meeting of senators.
"Come on in, John," Obama shouted.
A red-faced McCain turned around and headed out to chuckles and then applause from the Democrats. "My mistake," McCain said.
To the far-right in the Republican Party, there is no mistake about it. In a party that generally brooks no compromise with Obama, McCain is the compromiser-in-chief. He's negotiating with the White House.
And he's in-their-face about it, having recently described as "wacko birds" two of the right's most uncompromising heroes, Senators Ted Cruz and Rand Paul.
A recent article in the conservative National Review described McCain as "Obama's secret weapon" and a "lethal threat" to a Republican victory in the budget battles this fall.
If Obama somehow wins an upcoming fiscal showdown with Republicans, the president may have McCain to thank.
The 2008 Republican presidential nominee, the man Obama beat, is now engaged in talks with a half-dozen fellow Senate Republicans and the White House aimed at trying to craft a bargain on critical fiscal issues to avoid a nasty standoff in the fall that could lead to a government shutdown or a government default, or both.
Obama already owes McCain for brokering the deal last month that allowed the president to overcome Republican objections and fill seven top administration jobs.
And McCain joined four Democrats and three Republicans in drafting the Senate's landmark bill to overhaul U.S. immigration laws, a top Obama priority.
So popular has the Arizona Republican become among Democrats that Senate Majority Leader Harry Reid - who in 2008 said, "I can't stand John McCain" - now describes him as a "great senator," right up there with the late Ted Kennedy, the Democratic "lion of the Senate" who became a bridge between parties after his own presidential ambitions were crushed.
"He's become our go-to guy," a senior Democratic aide said of the 76-year-old white-haired senator.
That's an exaggeration. McCain remains a critic of Obama's handling of the deadly attack on a U.S. diplomatic mission in Benghazi, Libya, last September and of what he considers the "disgrace" of Obama's non-intervention in Syria.
But on domestic issues, McCain has emerged as one of the top deal makers in Washington, a Republican Obama can do business with.
MOVING TOWARD THE MIDDLE
McCain said in an interview that he has been able to work more with Obama in part because the president has moved toward the middle in his second term and become more willing to negotiate.
McCain has also moved toward the middle after having veered toward the right in 2010 when he survived a Republican primary challenge and won election back home in Arizona to a fifth, six-year Senate term.
Two unrelated events have helped as well: McCain's new friendship with Charles Schumer, the third-ranking Senate Democrat who worked with McCain on the immigration bill, and Obama's choice of a McCain friend, Denis McDonough, as White House chief of staff.
The three have clicked.
At a time when the top congressional Republicans, Senate Minority Leader Mitch McConnell and House of Representatives Speaker John Boehner, rarely communicate with the White House, McCain does.
He said he and McDonough talked three times a day during the negotiations over Obama's stalled nominations.
"With all due respect to his predecessors, Denis knows how the Senate works," McCain said of McDonough, whom he knows from McDonough's days as a senior aide to former Democratic Senator Tom Daschle.
"I will continue to pound him (Obama) on Syria. I will continue to pound him on Benghazi," McCain said. "I will continue to pound him on a number of things, particularly on national security.
"But at the same time, I can find common ground with the president on certain issues," McCain said.
"The president won't owe me anything, but he may have a sense of satisfaction from being able to achieve some of the things that we ostensibly came here to do," McCain said.
At the same time he is helping Obama, McCain may be helping himself.
He likes attention as much as any other politician in Washington - and more than many - and there's no better way to get it than to talk about wacko birds.
In a city that rarely makes a deal, his recent feats of deal making have put him at the center of the action in Washington, enhancing his value as one of television's favorite political guests.
"I think John has more job satisfaction now than any time in his career," said Republican Senator Lindsey Graham, one of McCain's closest friends.
"He wanted to be president, but it didn't work out," Graham said. "It takes a while to get over it. Right now, John realizes that the country is in trouble and believes he can help."
"He's back in his groove," Graham said.
(Reporting by Thomas Ferraro; Editing by Fred Barbash and Eric Beech)