PORTLAND – Connecticut Sen. Joseph Lieberman said he is endorsing Sen. Susan Collins in her 2008 re-election bid and will campaign on her behalf.

Lieberman, a former Democrat turned independent, is returning the favor to Collins, a Republican who supported and campaigned for him in his 2006 election in Connecticut. Although Lieberman is an independent, he continues to caucus with Democrats.

Lieberman’s announcement comes amid speculation about who will challenge Collins for the Senate seat in the 2008 election. Democratic Rep. Tom Allen has signaled he might run for the seat, but has not announced he will do so.

Lieberman told the National Journal’s “CongressDaily” on Thursday that he is ready to support Collins even though he doesn’t know who the Democratic nominee will be. He said his political action committee has contributed $5,000 toward Collins’ campaign.

Collins, a two-term senator, has served closely with Lieberman on the Senate Homeland Security and Governmental Affairs Committee.