Joe Lieberman is fast becoming the Democrats' public enemy No. 1.

WASHINGTON (AP) — Joe Lieberman is fast becoming the Democrats' public enemy No. 1.

The four-term Connecticut senator, who came tantalizingly close to being Al Gore's vice president in 2000, not only has been campaigning for his pal, presumed Republican nominee John McCain, now he's publicly criticizing the Democrats' standard-bearer, Barack Obama. Lieberman has strayed before, most notably switching from Democrat to independent in 2006 to hold onto his Senate seat after a Democratic primary loss.

But the latest betrayal has upset Democrats, who often answer in clipped but polite tones when asked about Lieberman. The reason:The independent still caucuses with the Democrats on most issues except the Iraq war, and he holds their slim political majority in his hands.

"There's a commonly held hope that he's not going to be transformed into an attack dog for Republicans," said Sen. John Kerry, D-Mass., an Obama supporter.

Lieberman has wasted no time in questioning Obama's positions on Iran and Israel, two topics on which Lieberman and McCain agree.

Just one day after Obama clinched his party's nomination, Lieberman joined Republicans on a McCain campaign teleconference call assailing Obama following his foreign policy address to a leading Jewish group.

Lieberman accused Obama of blaming U.S. policies for "essentially sort of strengthening" Iran.

"If Israel is in danger today, it's not because of American foreign policy, which has been strongly supportive of Israel in every way," he said. "It is not because of what we have done in Iraq. It is because Iran is a fanatical terrorist, expansionist state."

Later that day, during a budget vote in the Senate, Obama led Lieberman to a corner of the Senate floor for a pointed private conversation. Without elaborating, Obama told reporters the chat was about politics. Senate Majority Leader Harry Reid, D-Nev., had a similar private conversation with Lieberman.