"While I believe that I will win the Aug. 8 primary, I know there are no guarantees in elections," Lieberman told reporters on the steps of Connecticut's statehouse. "No one really knows how many Democrats will come out to vote on what may be a hot day in August."
Once a Democratic stalwart and the party's 2000 vice presidential nominee, Lieberman has fallen into disfavor from some Democrats for his support of the Iraq war and his perceived closeness to President Bush.
He has also been target by liberal blogs, which have given a boost to his challenger and bill the race as a chance to send a message to the Democratic establishment.
The ActBlue.com Web site, which helps Democrats set up fundraising campaigns for candidates, has helped bring challenger Ned Lamont nearly $233,000 in contributions as of last week. Lamont also received about $70,000 in contributions from the liberal group Democracy for America by way of the Internet.
Lieberman, running for a fourth Senate term, said his decision to collect signatures was influenced by Lamont's wealth and concerns of a low primary turnout.
"What if my opponent, who says he is worth somewhere between $90 million and $300 million, decides to write bigger and bigger checks in the last weeks of the campaign?" he asked.
Lamont, a multimillionaire owner of a cable television company with little political experience, has so far spent $1.5 million of his own money on his campaign. He has called Lieberman a Republican lapdog and accused him of straying from his Democratic roots.
At a separate news conference Monday, Lamont said Lieberman's decision may split the party and hurt Democrats running for U.S. House seats in Connecticut.
"I can see why he's doing it," Lamont said. "He's worried about the fact that we've an awful lot of support right now."
Lieberman said he wants to take his case to the entire state should he lose the primary.
"My opponent in the Democratic primary is asking Democrats to vote against me because of position on one issue, Iraq," Lieberman said. "I'm asking Democrats in the primary in August and the general election in November to consider my total record."
Quinnipiac University polls show Lamont's support among Democrats increasing from 19 percent in May to 32 percent in June. Lieberman's support in the same period fell from 65 percent to 57 percent.
Lieberman maintains high ratings among Republicans and unaffiliated voters, however, and unaffiliated voters are the state's largest voting bloc, followed by Democrats and Republicans.
The poll also found that if Lieberman runs an unaffiliated campaign, he would win with 56 percent of the vote, compared with 18 percent for Lamont and 8 percent for Republican Alan Schlesinger.
Schlesinger predicted that he would win a three-way race.
"Joe Lieberman first said that he's the best Democrat. Then he tells some people he's the best Republican," Schlesinger said. "So what's he going to do now, tell the people he's the best independent?"
Lieberman has until Aug. 9 — the day after the Democratic primary — to collect 7,500 signatures from registered voters to land on the November ballot as an unaffiliated candidate. Experts have said his petition drive during the primary could further annoy Democrats who already question his party loyalty.