WASHINGTON - There was no conciliatory phone call, no heart-to-heart talk to soothe the tensions. No one knows exactly when President Barack Obama and Republican Sen. John McCain went from bitter rivals in the 2008 presidential campaign and foes over health care and national security to bipartisan partners.

Yet in recent months, an alignment on high-profile domestic issues - not to mention an eye on their respective legacies - has transformed Obama and McCain into Washington's most unexpected odd couple. The Arizona senator is a regular visitor to the West Wing and in near-daily contact with senior White House officials.

McCain, in an Associated Press interview, said that he and Obama "trust each other." White House chief of staff Denis McDonough, among the Obama advisers who speak regularly with McCain, praised the lawmaker as a "refreshing" partner who "welcomes a debate and welcomes action."

Like any good business arrangement in the nation's capital, the secret to the new Obama-McCain alliance ultimately comes down to this: Both sides believe that working together is mutually beneficial and carries little political risk.

For Obama, the senator has become a rare Republican backer of important elements on the president's second term agenda, including immigration overhaul, stricter background checks for gun buyers, and perhaps a fall budget deal.

In return, McCain has secured increased access to the White House and an opportunity to redeem his reputation as a Capitol Hill "maverick." That image was tainted when McCain tacked to the right during his failed 2008 presidential run against Obama.

"I've told the people of Arizona, I will work with any president if there are ways I can better serve Arizona and the country," McCain said. "That seems to be an old-fashioned notion but it's the case."

Indeed, the level of attention lavished on a functional working relationship between the Democratic president and the Republican senator underscores how rare such partnerships have been during Obama's tenure.

Lawmakers, including some Democrats, long have chafed at Obama's distant dealings with Capitol Hill and his supposed lack of understanding about how Congress operates.

It's unlikely that Obama and McCain's partnership will lead to a larger detente between the White House and congressional Republicans. While McCain may have sway over some like-minded members of the Senate Republican caucus, he has considerably less influence with his party's more conservative wing, particularly in the GOP-controlled House.

Still, the White House is hopeful that forging policy breakthroughs with McCain and other Senate Republicans will isolate the House GOP and perhaps persuade them to act.

The first test of that strategy probably will be the White House-backed immigration overhaul. McCain helped write and shepherd the bill through the Senate last month. Its future in the House is deeply uncertain.

The administration also will try to work with McCain ahead of impending budget battles, McDonough said, given that the senator and the White House agree there is a negative impact from across-the-board federal budget cuts, particularly on the military and defense industry.

McDonough said it's not just a shared view on policy that has made McCain an attractive partner to Obama on these and other issues. It's their mutual disdain for Washington meetings that never move beyond the standard talking points.

"Part of what's great to work with him is his impatience with that," McDonough said. "You can kind of get into the meat of the matter very quickly"

Obama and McCain were never close during the president's brief tenure in the Senate. While McCain is a creature of Capitol Hill, Obama largely saw Congress as a stepping stone to bigger things. The relationship deteriorated during frequent clashes in the 2008 presidential campaign, and it often appeared during Obama's first term like it would never recover.

In 2010, the two sparred during a televised negotiating session on health care. McCain chastised Obama for brokering deals behind closed doors, to which the president snapped, "We're not campaigning anymore. The election is over."

McCain replied: "I'm reminded of that every day."

White House advisers still bristle over McCain's accusations that the administration covered up details of last year's deadly attacks on Americans in Benghazi, Libya, as well as his relentless criticism of former U.N. Ambassador Susan Rice's role in that alleged effort.

McCain's criticism contributed to Rice's decision to withdraw from consideration as Obama's secretary of state. She now serves as White House national security adviser, a post that does not require Senate confirmation.

McDonough acknowledged that McCain's role in keeping the Benghazi controversy alive has been a source of frustration. But he credited the senator with largely shelving his criticism of Rice once she joined the White House staff.

"The way he's worked with her since she became national security adviser speaks to his interest in making sure that even where we disagree, we're finding a way to work together when we can," McDonough said. "I know the president has appreciated that."

McCain said his stronger ties with the president on domestic issues won't keep him from challenging the president on national security issues, including Syria, where McCain backs a more aggressive U.S. response than does the administration. But he said there's a way to strike an appropriate balance.

"He is the president of the United States," McCain said. "You can strongly disagree and still be respectful."