(CNN) -- Sen. Joe Lieberman is crossing party lines and endorsing Republican Sen. John McCain for president.
Sen. Joe Lieberman, I-Connecticut, is endorsing Sen. John McCain, R-Arizona, for president.
"Being a Republican is important. Being a Democrat is important. But you know what's more important than that? The interest and well-being of the United States of America," the Democrat-turned Independent said in announcing his decision Monday morning in New Hampshire.
"Let's put the United States first again, and John McCain is the man as president who will help us do that," he said.
Lieberman, the Democrats' vice presidential nominee in 2000, said the next president needs to "break through the reflexive partisanship that is poisoning our politics today and stopping us from getting anything done in Washington." Watch Lieberman explain his move »
In accepting Lieberman's backing, McCain said, "It is one of the great honors of my political career to have known him, to have worked with him and to have received his endorsement."
The Connecticut senator decided to endorse McCain because he considers him "the most capable to be commander in chief on day one of his administration, and the most capable of uniting the country so that we can prevail against Islamic extremism," a Lieberman aide said earlier.
The Lieberman aide insists the senator does not see this as a "commentary on or an endorsement of the Republican Party, only the person."
The endorsement could help emphasize McCain's experience on national security issues, demonstrate his ability to work across party lines and perhaps help persuade Independent voters in New Hampshire to support his 2008 presidential bid, as many of them did during his first campaign in 2000.
McCain also picked up key endorsements this weekend from two of the most influential papers for voters in Iowa and New Hampshire, The Des Moines Register and The Boston Globe.
Lieberman, who broke with the Democratic Party over Iraq, won re-election as an Independent after losing the party's Senate nomination to an anti-war challenger in 2006.
He still caucuses with the Democrats, giving them their razor-thin majority in the chamber. Lieberman chairs the Homeland Security & Governmental Affairs Committee, and he still attends weekly Democratic strategy meetings.
But his adviser said the endorsement appears to be a "hangover" from the 2006 race, when top Democrats backed the Democratic nominee, Ned Lamont, rather than support Lieberman's Independent candidacy.
"I have the greatest respect for Joe, but I simply have to disagree with his decision to endorse Senator McCain," Senate Majority Leader Harry Reid, D-Nevada, told CNN in a written statement.
Lieberman's office called Reid's office Sunday to inform the Democratic leader of his decision to endorse a Republican, a Reid spokesman said.
In response to the endorsement, Al From, founder and CEO of the Democratic Leadership Council, said, "I am very saddened by Senator Lieberman's choice, and profoundly disagree with it. We need to elect a Democratic president in 2008."
Lieberman and McCain co-sponsored the 2002 resolution that authorized the U.S. invasion of Iraq the following March, and Lieberman has remained a supporter of the nearly 5-year-old war in Iraq at a time when Democratic leaders are trying to bring the conflict to an end.
He had not planned to endorse anyone until after the primary season, but McCain asked for his endorsement a few days after the two men returned from a Thanksgiving trip to Iraq together, the aide said.
A CNN/WMUR poll released last week found McCain tied for second in the New Hampshire, which hosts the first primary of the 2008 presidential race in January.
He and former New York Mayor Rudy Giuliani both rated 19 percent support among New Hampshire Republicans, while Mitt Romney, the former governor of neighboring Massachusetts, led the race with 32 percent.
The poll of 354 likely Republican primary voters was conducted from December 6 through December 10. It has a margin of error of plus or minus 5 percentage points. E-mail to a friend
CNN correspondents John King and Dana Bash contributed to this report.