Relations have been strained ever since Barack Obama endorsed Ned Lamont over Lieberman when the latter challenged the former in 2006. ~Marc Ambinder

That’s a bit surprising, since Obama endorsed Lieberman during the Democratic primary race itself long before most of his colleagues did.  (Obviously Obama wasn’t going to go against his party’s nominee to support a certain sore loser after the primary.)  It’s a strange sense of loyalty that demands that a colleague keep supporting your campaign after you lose a primary and decide to break with your party. 

This is from the AP story on Obama’s backing of Lieberman at the time:

“The fact of the matter is, I know some in the party have differences with Joe. I’m going to go ahead and say it,” Obama told the 1,700-plus party members who gathered in a ballroom at the Connecticut Convention Center for the $175-per-head fundraiser.

“I am absolutely certain Connecticut is going to have the good sense to send Joe Lieberman back to the U.S. Senate so he can continue to serve on our behalf,” he said.

Obama received widespread attention for his keynote address at the 2004 Democratic National Convention, delivered while he was still a state senator.

Lieberman became Obama’s mentor when Obama was sworn into the Senate in 2005 [bold mine-DL]. They stayed close at Thursday night’s event, too, entering the room together and working the crowd in tandem.

Frankly, this is the mentor-pupil relationship that Obama’s critics ought to spend time focusing on.