PORTLAND – In 2006, Republican U.S. Sen. Susan Collins of Maine campaigned for Connecticut colleague Joseph Lieberman, the former Democrat who won re-election as an independent after losing his party’s primary.

Now, as Collins faces what many see as a tough re-election challenge from Democrat Tom Allen, Lieberman has returned the favor by backing Collins and raising money for her campaign.

Political experts and the candidates themselves are split on how Lieberman’s involvement in the 2008 race could affect the outcome.

Some suggest that having a former Democrat in her corner could rally swing voters to the two-term senator by distancing her from the policies of President Bush.

“It’s better to run as a Joe Lieberman Republican than a George Bush Republican,” said Stuart Rothenberg, a political handicapper and author of the Rothenberg Political Report.

Since most endorsements are predictable, Rothenberg said, getting the nod from a former Democrat who was his party’s vice presidential nominee in 2000 is a plus for a moderate Republican.

Others suggest that the endorsement could backfire by generating more support for Allen from liberal Democrats who use the Internet to organize and raise money, and who regard Lieberman as a pariah because of his advocacy of the war in Iraq.

“Nobody fires up the Netroots more than Joe Lieberman, not even President Bush,” said Amy Walter, editor of the political newsletter The Hotline.

Allen said that on balance, the endorsement doesn’t hurt his campaign. “For the Democratic base, Lieberman’s advocacy of the war in Iraq and, more recently, Iran, has made him a bit of a lightning rod,” the six-term congressman said.

Collins said she “appreciates Lieberman’s willingness to work across party lines,” while noting that “Joe and I don’t agree on every issue. He has a much more aggressive, hawkish position on the war.”

Collins and Lieberman have forged a close relationship while serving on the Homeland Security and Government Affairs Committee and on fact-finding trips to South Asia and the Middle East.

“Really, the relationship grows out of our work together on the committee,” Lieberman said. “Ultimately, we’ve developed a lot of trust.”

After being asked by Collins for his support, Lieberman announced in April that he would endorse her. He has since raised money for her campaign in Hartford, Conn., and in Washington, D.C., and contributed $10,000 from his political action committee.

Ned Lamont, the Democrat who beat Lieberman in the 2006 primary only to lose in the general election, helped raise money for the Maine Democratic Party, appearing last summer at its annual Edmund Muskie Lobster Bake.

Lieberman said he hopes his endorsement doesn’t backfire and that it helps Collins’ campaign.

“I’m disappointed to become such a target from liberal Democrats,” he said. “Sometimes in our politics today, partisanship overcomes friendship, and that’s a value that I don’t respect.”