( 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 .
Sen. Joe Lieberman was Democrats ' vice presidential nominee in 2000 . Lieberman broke with Democrats and became an Independent . McCain picked up endorsements from The Des Moines Register , The Boston Globe . McCain tied for second among GOP candidates in New Hampshire .