"I have not foreclosed the option," Lieberman said at a news conference at the Capitol. "If I wanted to run as an independent, I would. I'm running as a Democrat. I've been a Democrat all my life."
Ned Lamont, a Democratic activist and anti-war candidate, is challenging Lieberman for the party's nomination this year. He has been garnering support from some Connecticut Democrats dissatisfied with Lieberman's stance on Iraq and his perceived closeness with President Bush's administration.
The party will endorse a candidate at a convention on May 20.
Lieberman, the Democratic vice presidential candidate in the 2000 election, said he is not upset with fellow Democrats for opposing the war in Iraq. He said he also has criticized certain operations of the war and the Bush administration's handling of postwar Iraq.
"I feel very strongly the world is safer without Saddam Hussein in power. We have to complete the job in Iraq," Lieberman said.
He said Democrats should also consider his record on the environment, economic development and his support for civil rights and civil liberties.
Lamont did not return a call seeking comment.
Paul Streitz, who is seeking the Republican nomination, said he would gain if Lieberman is upended by Lamont and runs as an independent.
"A Republican would win, obviously," he said. "It would split the Democratic vote."
Howard L. Reiter, chairman of the political science department at the University of Connecticut, said an incumbent senator running without a party is not unprecedented.
Sen. Thomas Dodd was censured by the U.S. Senate in 1967 for financial misconduct and was denied renomination by the Democratic Party, which backed Joseph Duffey. Dodd ran as an independent and lost the general election to Republican Lowell Weicker, who was defeated by Lieberman in 1988.
"That's the precedent, but you have to go back 36 years," Reiter said.
Lieberman, who became a national political figure as his party's vice presidential nominee in 2000, unsuccessfully sought the presidential nomination four years later.
He has burnished a reputation as a maverick who bucks his party. He was one of the few Democrats to chide then-President Clinton during the Monica Lewinsky scandal and has been a prominent critic of the sex and violence depicted in video games and by Hollywood, a major source of campaign money for Democrats.
Lieberman also voted for the 1991 Gulf War, casting the fight in moral terms.
He was in Hartford on Monday to speak at a conference on global warming.
By Stephen Singer