Connecticut Sen. Joe Lieberman, a Democrat who ran as an independent last election after losing his primary re-election, endorsed Republican Sen. John McCain on Monday.

Lieberman, who ran for vice president in 2000 alongside Al Gore, said McCain possessed the greatest ability to break gridlock in Washington.

"There are some things more important than the political parties. One is friendship and the other is I think this guy is the best of all the candidates to unite our country and cross political lines so we can begin to finally solve some of the political problems that we have in this country and to lead us against the war versus Islamic elitist terrorism," Lieberman told FOX News.

McCain said the endorsement shows he can reach across the aisle, and suggested he would like to have Lieberman in his administration.

"In an era when politicians so often put political expediency ahead of conviction, Joe Lieberman has stood up for what he believes is in America's interest. He has stood tall against the prevailing winds on national security and understands why we must succeed in Iraq and in the broader war against radical Islamic extremism. He is a principled leader, a good friend, and I am proud to have his support," McCain said in an announcement in Hillsborough, N.H.

A senior Lieberman aide told FOX News on Sunday that Lieberman was approached by McCain, a longtime friend and ally, a couple of weeks ago and asked for the endorsement.

The two had just traveled together, once again, to Iraq for the Thanksgiving holiday. Lieberman has long said he would let the process "play itself out" -- meaning, not endorse a candidate early, but according to the aide, "Lieberman just thought McCain was clearly the most qualified candidate (of the entire '08 line-up) to be commander in chief from day one so rather than just observe from the sidelines, the senator now hopes to actually influence the process."

The aide said Lieberman was not courted by anyone else in the race.

"I think McCain is the only one who asked for the senator's endorsement," he added.

In a pre-emptive comment against the questions that will inevitably come and that have swirled around the senator since his own contentious 2006 re-election, when he was knocked out of the Democratic primary by a political neophyte because of his pro-Iraq position, Lieberman is not switching parties.

"This is in no way an endorsement of the (Republican) Party, just the man," the aide said, adding that McCain did not ask Lieberman to join his ticket in the vice presidential slot.

Lieberman "just wants to serve as a U.S. senator, nothing more," the aide said.

The McCain-Lieberman relationship is decades old. The two worked closely together on Kosovo and the Balkans issues in the early 1990s, and "really cemented their friendship" by hosting an annual trip to Munich each year for the Conference on Security Policy. Both men also pushed legislation, against the Bush administration's wishes, that eventually led to the establishment of the Sept. 11 commission.

"I have many friends running for president this year and they're fine candidates, but what I'm saying is John McCain, in my opinion, based on his record and all the work we've done together on keeping the military strong ... creating a 9/11 commission, in all of those cases I saw him rise above the negative, smallness partisanship to get something done," Lieberman told FOX News.

Lieberman made his decision one week ago, but due to some scheduling issues, it had to wait until now.

McCain is polling second to Mitt Romney in New Hampshire, 33-20, according to the latest FOX News-Opinion Dynamics poll. That poll, taken Dec. 11-13, of 500 likely New Hampshire Republican voters had a 3 percent margin of error.

McCain won the New Hampshire Republican primary in 2000. He has scored the endorsement of the Manchester Union-Leader, New Hampshire's only statewide newspaper, and on Sunday won the backing of The Boston Globe in the state where Romney used to be governor.

"As a lawmaker and as a candidate, McCain has done more than his share to transcend partisanship and promote an honest discussion of the problems facing the United States. He deserves the opportunity to represent his party in November's election," wrote The Globe's editorial board, which admittedly does not agree with McCain on much.

FOX News' Trish Turner contributed to this report.