They've come a long way from "Shame on you, Barack Obama" and "You're likeable enough, Hillary."

Hillary Clinton and Barack Obama/REUTERS

President Obama and Secretary of State Hillary Clinton are sitting for a rare joint interview set to run Sunday on CBS' "60 Minutes", the latest sign that the once-hostile relationship between the two has thawed considerably over the past four years.

"I think there was a reason that the 2008 fight for the Democratic nomination was as long and intense as it was," said Patti Solis Doyle, who managed Clinton's campaign. "These are two brilliant, formidable people who have, after working on the same team, developed a clear mutual respect and admiration."

Another source familiar the relationship between the two put it this way: "As amazing as it seems, Obama came to see Clinton as like himself -- a no drama adult in a city he thinks is full of posturing phonies.  Who woulda thunk it?"

It's not clear whether the relationship between Obama and Clinton is one borne -- and built -- on admiration and commitment to common causes or a true and genuine friendship (we would lean toward the former) but it's beyond debate that the two have moved well beyond the divisiveness of the 2008 campaign.

"Secretary Clinton has been first among equals as part of the President's team, which is a long way from being first among equals on the presidential campaign trail," said Stephanie Cutter, deputy campaign manager for Obama's re-election race.

One source close to the situation suggest that the relationship's evolution from rivals to friends stems from simply spending lots (and lots) of time together over the past four years -- particularly on foreign travel.  Familiarity, in this case, bred camaraderie not contempt, it seems.

When the detente began exactly is somewhat hard to trace although one Democrat suggested that it all started when Hillary and Bill Clinton both spoke at the 2008 party convention.

"All of [Obama's] senior people were panicked" about the Clinton speeches undercutting the party's nominee, recalled the source.  "Both Clinton convention speeches were unstintingly generous in their praise of Obama and made a direct appeal to Clinton voters to support him....I think that was the beginning of the change in Barack Obama's attitude."

What's also evident from the various post-mortems written about the 2008 race -- and there have been many -- is that while the contest was clearly pointed and nasty, much of the deeper-seeded ill will was between the staffs (and donors) of the two campaigns rather than the candidates themselves.

What remains to be seen is how the relationship holds up over the next few years as Obama seeks to burnish his record and Clinton contemplates running for president again.  Complicating that dynamic is Vice President Joe Biden who Obama has praised lavishly for the job he has done and who has made little attempt to hide his interest in a presidential bid in his own right in 2016.

Biden would, obviously, want the strong endorsement of the sitting president who will almost certainly remain an extremely popular figure within the Democratic party over these next four years. But, endorsing Biden in a race that also includes Clinton would be a very tough thing for Obama to do. Of course, it's likely that this whole, delicious debate will be moot as either Biden or Clinton (or both) will decide not to run for president in three years time.

Regardless of what's next for Barack and Hillary, what has happened over the past six-ish years between the two of them is a remarkable political tale. They are now, by all accounts, friends. Not the sort of friends who pal around together in their off hours - neither Obama nor Clinton seem to be in the market for anymore of those types of friends -- but two people who, over time, have found plenty to admire in the other.  And that is saying something given where this all started way back in 2007.