Lieberman, who ran and won as an independent after losing the Democratic nomination for U.S. Senate in Connecticut in 2006, still caucuses with the Democrats. But he is campaigning for Republican U.S. Sen. John McCain for president and has agreed to speak at the Republican convention.

While a Lieberman spokesman called the conversation Wednesday "cordial and friendly" and an Obama spokesman called it "private and friendly," sources told ABC News that Obama told his colleague he had gone too far in criticizing him.

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

Obama, the junior senator from Illinois, supported Lieberman in the 2006 primary and stayed out of his race with the Democratic candidate, Ned Lamont.

Lieberman, like McCain, supports the Iraq War and has criticized Obama for saying he would talk to the governments of Iran and Syria.