WASHINGTON (CNN) -- If you had any doubt that Joe Lieberman marches to the beat of his own drummer, this should seal the deal. The Independent senator from Connecticut will help throw a Washington fundraiser next month for Sen. Susan Collins, the Republican from Maine.

Co-hosting the June 21 event with Lieberman will be another Republican: Sen. Arlen Specter of Pennsylvania.

Collins, who could face a tough re-election fight next year, is a top target for Democrats, who successfully defeated another moderate northeastern Republican senator in 2006, Sen. Lincoln Chafee of Rhode Island.

Lieberman, a long-time Democrat and his party's 2000 vice presidential nominee, lost the Democratic primary for a fourth term in the Senate last summer to businessman Ned Lamont, but mounted a successful general election bid as an Independent. He is one of two Senate Independents who caucus with the Democrats, giving the party their razor-thin 51-49 majority.