Sen. Lieberman will throw his support behind McCain, a senior GOP source tells CNN.
WASHINGTON (CNN) - Sen. Joe Lieberman, a Democrat turned Independent, will endorse Republican Sen. John McCain for president, officials close to both Lieberman and McCain familiar with the plan tell CNN.
Lieberman is planning to announce his support for McCain at an early Monday morning event in New Hampshire, but the campaign is keeping a close eye on a winter storm that could force it to be rescheduled.
The McCain campaign declined to comment on the source's account, and would not confirm it.
An aide to Lieberman tells CNN he 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."
The Lieberman aide insists the senator does not see this as a "commentary on or an endorsement of the Republican party, only the person."
Lieberman had not planned to endorse anyone until after the primary season, but McCain asked Lieberman for his endorsement a few days after the two men returned from a Thanksgiving trip to Iraq together, and Lieberman decided to do it, according to the same Lieberman aide.
Lieberman will continue to caucus with the Democrats.
Like McCain, Lieberman has been a vocal supporter of the Iraq war. For Lieberman, it is an issue that caused him to split with his own political party after losing the Democratic Senate primary in 2006. Lieberman refused to back down, and won reelection as an Independent.
Lieberman was the Democratic vice presidential nominee in 2000.
Lieberman's Democratic colleagues welcomed him back to the Senate, and he is the 51st vote that gives Democrats a razor thin majority in the 100 member chamber. Lieberman chairs the Homeland Security & Governmental Affairs Committee, and he still attends weekly Democratic strategy meetings.
"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 said in a statement to CNN.
Lieberman's office called Reid's office Sunday to inform the Democratic leader of his decision to endorse a Republican.
A longtime Lieberman adviser described it as a "hangover" from the 2006 campaign when Democrats, including Connecticut Sen. Chris Dodd, New York Sen. Hillary Clinton, and other longtime friends declared they could and would not support his reelection bid after he lost the primary. Instead, they backed the Democratic nominee Ned Lamont.
This endorsement could help emphasize McCain's national security standing, show he is able to work across party lines, and perhaps help persuade independent voters in New Hampshire to support his presidential bid.
– CNN's John King and Dana Bash