The state's Democratic registrar rejected a petition by peace activists to strike Lieberman from the party rolls on Thursday. State law allows for expulsion from the party if the member runs for office under another political banner.
The senator lost the Democratic primary and is running in the general election as part of a new party, Connecticut for Lieberman.
Had Registrar Sharon Ferrucci agreed to the petition, she could have scheduled a hearing in which Lieberman would have to explain why he should remain in the party that nominated him for the vice presidency in 2000.
The authority to expel a party member "should be rarely and cautiously exercised," Ferrucci said.
"The Democratic Party is founded upon principles of inclusion and diversity of opinion and the promotion of debate," Ferrucci said. "I do not intend to summon anyone enrolled in my party to defend the good faith and bona fide nature of their affiliation with the Democratic Party."
Lieberman lost the Aug. 8 primary to businessman Ned Lamont, who criticized Lieberman for his support of the Iraq war and said he was too close to Republicans and President Bush.
Since losing the primary, most Democratic Party leaders have abandoned Lieberman and endorsed Lamont. A Quinnipiac University poll last week showed Lieberman with a 12-point lead over Lamont, although another poll this week shows the race much closer. Republican Alan Schlesinger had only single-digit support in both polls.