Sen. Joe Lieberman, the self-described "Independent Democrat" who caucuses with the Democratic party in the Senate even though he has endorsed Sen. John McCain, R-Ariz, got some tough talk from Sen. Barack Obama, D-Illinois, yesterday about his advocacy for the presumptive Republican presidential candidate and the general tone of the campaign, Democratic sources tell ABC News. Returning to the Senate after his securing the Democratic presidential nomination, Obama and Lieberman greeted each on the Senate floor in the Well as they were voting on the budget resolution.

    They shook hands. But Obama didn't let go, leading Lieberman - cordially - by the hand across the room into a corner on the Democratic side, where Democratic sources tell ABC News he delivered some tough words for the junior senator from Connecticut, who had just minutes before hammered Obama's speech before the pro-Israel group AIPAC in a conference call arranged by the McCain campaign.

    The two spoke intensely for approximately five minutes, with no one able to hear their conversation. Reporters watched as Obama leaned closely in to Lieberman, whose back was literally up against the wall.

    Neither party is officially talking. But while Lieberman spokesman Marshall Whitman says the conversation was "a cordial and friendly discussion" and Obama campaign spokesman Bill Burton says it was "private and friendly," Democratic sources tell ABC News that the conversation was a stern rebuke to Lieberman for his criticism of the presumptive Democratic presidential nominee on the conference call, as well as a discussion about how far Lieberman is willing to go in his advocacy of McCain, and the tone of the campaign.

    "It's one thing to support McCain," said one Democratic source, "but many think Uncle Joe has gone too far."

    Obama campaigned for Lieberman in 2006 when he was challenged (and ultimately defeated) in his primary race for his Senate seat. When Lieberman opted to run as an independent, Obama wrote a supportive email endorsing Democratic nominee Ned Lamont, but he did not appear in person for him, unlike other Democrats, such as Sen. Chris Dodd, D-Conn.

    On the McCain conference call yesterday, Lieberman congratulated Obama "in securing the Democratic nomination and to express my own hope as a supporter of John McCain that this will be a civil and constructive campaign debate from here to November."

    The only Orthodox Jew in the U.S. Senate then criticized the White House hopeful's speech to the Jewish pro-Israel lobby AIPAC, saying, "I would say respectfully that I thought in this speech that there was a disconnect between things Senator Obama said today in particularly with regards to Iran and things that he has said or done earlier either in the campaign and senate. To be specific, I was troubled earlier in the year during the campaign season when Senator Obama referred to, I guess compared Iran and other rogue and terrorists states to the Soviet Union and minimized the threat represented by Iran. I think that is wrong."

    Lieberman also criticized Obama for voting against an amendment he offered with Sen. Jon Kyl, R-Ariz., that designated Iran's Revolutionary Guard a terrorist group, and included other language that Obama said linked the war in Iraq to Iran in a way that troubled him. "Senator Obama opposed it saying it was saber rattling and referring to the possible threat of military force," Lieberman said. "But if you look at the Kyl-Liebermann Amendment as it was passed, it has none of that in it, regarding military action. I was hoping and I still hope that he will say that that vote was a mistake, and that he would support that resolution."

    "Obama today argued that American foreign policy in recent years has essentially sort of strengthened Iran," Lieberman continued. "At one point, he almost seems to suggest that it helped to elect us Ahmadinejad, and has made Israel safe. I just disagree with that. Iran elected Ahmadinejad for their own reasons. If Israel is in danger today, it's not because of American foreign policy which has been strongly supportive of Israel in every way, it is not because what we have done in Iraq, it is because Iran is a fanatical terrorist, expansionist state and has a leader and a leadership that constantly threatens to extinguish the state of Israel."

    "Its a difficult situation," Sen. Dick Durbin, D-Illinois, the Senate Democrats' Assistant Majority Leader and a major Obama backer, told reporters Wednesday, according to Roll Call. "Joe is my friend ... but I hope he doesn't become the lead attack dog. Of course it's a concern when someone in your Caucus is supporting the other party's candidate. Let's not try and sugarcoat it."

    Lieberman agreed to caucus with the Democrats, who need his vote in the narrowly-divided Senate, in order to maintain power. But the Nutmeg stater is testing the patience of Democratic leaders by endorsing McCain and agreeing to speak at the Republican National Convention in September. Senate Majority Leader Harry Reid, D-Nev., told MSNBC they would "watch very closely" how far Lieberman takes his advocacy.

    But Obama may feel Lieberman has already taken it too far.

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    Jake Tapper is ABC News' Senior National Correspondent based in the network's Washington bureau. He writes about politics and popular culture and covers a range of national stories.