ABC News' Ron Claiborne and Bret Hovell Report: Sen. Joe Lieberman leapt across party lines Monday to endorse Sen. John McCain's bid for the Republican presidential nomination.
"I know it's unusual for a Democrat to be endorsing a Republican," Lieberman conceded, speaking alongside McCain at a Hillsborough, N.H. American Legion post.
"Political parties are important in our country. But they're not more important than what's best for our country, they're not more important than friendship," Lieberman said.
Lieberman, who was his party's nominee for vice president in 2000, lost in the 2006 Democratic Sentate primary in Connecticut in large part because of his support for the surge in Iraq. He came back to win as an independent, and caucuses as a Democrat, allowing Democrats control of the chamber by the slimmest of margins.
Lieberman spoke about national security and climate change, two issues that he and McCain have worked together on in the Senate.
"John McCain is not only the strongest on security of all of the candidates running for the presidency, but he's the real change candidate in this race," Lieberman said.
Speaking with reporters after the event, McCain talked about the uniqueness of Lieberman's endorsement.
"I think it's courageous when anyone crosses party lines to support a candidate from another party," McCain said. "The easiest thing for Joe to do would have just sat on the sidelines in this campaign."
McCain had said during the event that people were calling out for such bipartisanship.
"At town hall meeting after town hall meeting people stand up and say 'why can't you all work together? Why can't Republican and Democrat work together for the good of this country?' I think that his endorsement shows that I can and we can," McCain said before introducing Lieberman.
McCain frequently speaks of Lieberman on the stump, often calling him "my favorite Democrat" when talking about Lieberman's support of the surge in Iraq. The two senators have been allies on the issue, and recently traveled together to Iraq over the Thanksgiving holiday.
The endorsement could help McCain win over critical New Hampshire independents, who carried him to victory there in 2000 over then-Governor George W. Bush.
Lieberman's endorsement also caps a weekend of positive news for the McCain campaign, which is gaining traction in the early primary state of New Hampshire.
Over the weekend, McCain's candidacy was endorsed by The Boston Globe and The Des Moines Register, and has been previously endorsed by the New Hampshire Union-Leader.
ABC News' Jake Tapper and David Chalian contributed to this report.