More recently, he is said to be bitter that most of his Senate Democratic colleagues deserted him after he lost his party's nomination for re-election in 2006.
McCain did not endorse his re-election bid that year, but he did not support the Republican nominee, either.
McCain campaign officials said Lieberman would be campaigning for McCain, who added that he would also act as an adviser.
It has been a good couple of days for McCain. Lieberman's backing follows his endorsement by the Boston Globe and the Des Moines Register. McCain, however, is far behind in Iowa and barely campaigning there.
But it is hard to gauge how much endorsements of any kind mean these days. As the Hillsborough event broke up, a middle-aged woman from Derry said she was an undecided independent, torn between McCain and Democrat Barack Obama.
Would Lieberman's endorsement influence how she votes? "No," she said.