Sen. Joe Lieberman said Tuesday he was caught "completely off-guard" by the revelation that former Vice President Al Gore was endorsing Lieberman rival Howard Dean for the Democratic presidential nomination.

"I was surprised," Lieberman acknowledged in a nationally broadcast interview, but he said, "I'm more determined than ever to fight for what I believe is right for my party and my country."

Asked on NBC's "Today " program about whether he felt betrayed by Gore, who chose him as his 2000 campaign running mate, Lieberman said, "I'm not going to talk about Al Gore's (search) sense of loyalty this morning.

"I'll tell you, I'll always remain grateful " to Gore for choosing him for the vice presidential nod last time around, Lieberman said.

Reminded that he had declined to announce his campaign for the 2004 nomination until Gore had made his own decision about running, Lieberman said: "I did what I thought was right. I couldn't run against the guy who gave me an opportunity to run for vice president."

Lieberman, shown a news clip from 2000 in which Gore extolled his virtues as a man who would be ready to step in as president in a national emergency, said he could not explain why Gore decided to support the former Vermont (search) governor.

"Well, you'd have to ask Al," he said, "because I'm the same person today I was when he said those very kind things about me ... to put me into position to be president in the case of an emergency."

Lieberman has been struggling in his bid for the party's nod. He is no longer competing in Iowa, which has party caucuses next month, and he trails both Dean and Sen. John Kerry, D-Mass., in New Hampshire.

The Connecticut senator said Gore's move surprised him because he believes Dean's position on key issues are at odds with Gore's.

"That's why I'm surprised here," Lieberman said. "Al Gore has endorsed someone here who has taken positions diametrically opposite" of the former vice president.

"What really bothers me is that Al is supporting a candidate who is so fundamentally opposed to the basic transformation that Bill Clinton (search) brought to this party in 1992," moving it to a more middle-of-the-road stance on economic policy and other areas, Lieberman said.

"So, Al Gore has to explain why he's endorsing somebody who I think will take our party and our country backward," the senator said.

Lieberman quickly added that he did not think that Gore's endorsement of Dean would harm Lieberman's efforts in New Hampshire.

"No way," he said. "The voters are a month and a half away from voting."

"Al Gore has only one vote in the primaries, particularly in New Hampshire, where voters are independent-minded. I don't think they're going to be controlled by what any politician or pundit says," he said.

Lieberman said he had spoken Monday night with Clinton, but declined to say specifically whether they had talked about Gore's endorsement of Dean.

"Our conversations are always private," Lieberman said.

Reminded that he had said publicly last week that if he were elected president, he might give Gore a top-ranking position in his administration, Lieberman replied, "I'd say that's less likely this morning."