The decision, while expected, changes the dynamic of an otherwise sleepy Senate campaign. It creates a dilemma for Democrats worried about losing the seat, and an opportunity for Republicans who say Lieberman's decision shows a lack of faith in Gore's prospects.
A July poll found Lieberman, a two-term Democrat, with a 57-point lead over his little-known challenger, Waterbury Mayor Philip Giordano.
Lieberman remains an overwhelming favorite, but if he should also win the vice presidency it would fall to the Republican governor to appoint a replacement senator.
Gov. John G. Rowland has said he would appoint a Republican who would serve until the next statewide election, scheduled for 2002. But state Democratic Party Chairman Ed Marcus has said the Democrat-controlled Legislature could call a special election as soon as 2001.
Republicans now control the U.S. Senate 54-46; Democrats are hoping to regain control in this election cycle.
And while Democrats in Connecticut and Washington acknowledge that Lieberman's departure could affect that balance, they urge him to run for both offices. Lieberman would be following the example of two prominent Texans: Lyndon Johnson and Lloyd Bensten, who ran for vice president and Senate in 1960 and 1988, respectively.
"If the outcome of Joe Lieberman being elected vice president is to endure a Republican appointee to the Senate for two years, that's a price I'm willing to pay," said state Senate President Pro Tem Kevin Sullivan, D-West Hartford.
A spokeswoman for U.S. Senate Minority Leader Thomas A. Daschle of South Dakota said Daschle respects Lieberman's decision to stay in the Senate race.
But privately some Democrats concede they would rather Lieberman step aside, especially since Giordano would be an underdog to several Democrats, such as Attorney General Richard Blumenthal.
Republicans have already begun accusing Lieberman of hedging his bets.
"It sends a signal nationally that he believes Gore's chances of winning are not good," said state House Minority Leader Robert Ward, R-North Branford.
Lieberman spokesman Dan Gerstein said the 58-year-old senator "has every confidence in the vice president. They're going to win."
While Lieberman considered dropping out, Gerstein said, he ultimately decided it "would cause a lot of instability and potentially hurt the party if he took his name off the ballot."
A withdrawal "causes a free-for-all in a condensed time frame," Gerstein said.
Giordano has urged Lieberman to drop out of the Senate race. His campaign chairman, Timothy Longino, said Thursday of Lieberman's decision: "That's his gain and Connecticut's loss."
Marcus sided with Grstein, saying that all Democratic candidates will benefit from having the popular Lieberman listed twice on the state ballot.
And while Blumenthal who has eyed a potential Senate race for years would be the clear front-runner, he is not the only prominent Democrat interested in the job, Marcus and others said. Former U.S. Rep. Barbara Kennelly, current Reps. Rosa DeLauro and Jim Maloney, and former White House aide and gubernatorial candidate Bill Curry are among possible candidates if the race were to open.
On a practical level, Lieberman's entry into the race means he is likely to spend less time campaigning in Connecticut than normal. But Lieberman, who has more than $3 million in campaign cash, is expected to saturate the airwaves with TV ads promoting his Senate candidacy.
And thanks to his vice presidential campaign, Lieberman will be more visible than ever, Democrats say.
"Everybody in the state knows by now that Joe Lieberman is running for vice president, and they also will know he's running for Senate," Marcus said. "I think it's a net plus."
That thinking could change. Lieberman could quit the Senate race as late as Oct. 27 and be replaced by another Democratic nominee. If October polls indicated he and Gore were ahead of Republicans George W. Bush and Dick Cheney, Lieberman might end his Senate campaign, said Douglas Schwartz, director of the Quinnipiac University poll.
"I suppose he could wait and see how things are going and pull out of the Senate race so Democrats can hold onto the Senate seat as well as hold onto the presidency," Schwartz said.