People close to both men insist their friendship is durable enough to withstand McCain’s barrage of criticism, and Kerry confidantes say the secretary of state shrugs off his antagonist’s potshots. Kerry has invited McCain to breakfast at the State Department, and occasionally speaks to him by phone.

But the bond has clearly frayed of late. One person who recently saw them together described their rapport as more cordial than brotherly.

“This has always been a volcanic marriage of two strong willed, proud guys who deeply respect each other, but sometimes drive each other absolutely crazy. It’s a real friendship, unlike so many of the cliched Washington variety,” says one close Kerry associate.

“But this phase has been jarring. Kerry has been surprised by McCain’s public tirades. It’s crossed some lines that Kerry himself is always careful never to cross,” the associate added.