(CNN) -- Mitt Romney suspended his bid for the Republican presidential nomination Thursday, saying if he continued it would "forestall the launch of a national campaign and be making it easier for Senator Clinton or Obama to win."
"In this time of war, I simply cannot let my campaign be a part of aiding a surrender to terror. This is not an easy decision. I hate to lose," the former Massachusetts governor said.
"If this were only about me, I'd go on. But it's never been only about me. I entered this race because I love America, and because I love America, in this time of war I feel I have to now stand aside for our party and for our country."
Romney made the announcement Thursday afternoon at the annual Conservative Political Action Conference in Washington.
With Romney out, Sen. John McCain is locked in as the front-runner in the GOP race.
Romney had won 286 delegates through the Super Tuesday contests, compared with McCain's 697.
The crowd booed when Romney mentioned McCain, saying, "I disagree with Sen. McCain on a number of issues." Watch as Romney bows out »
"But I agree with him on doing whatever it takes to be successful in Iraq, on finding and executing Osama bin Laden, and I agree with him on eliminating al Qaeda and terror worldwide," he said.
According to a senior McCain adviser, McCain called Romney and told him he "admired his speech today and that he was a tough competitor."
McCain also told Romney he looks forward to sitting down with him at the earliest opportunity. McCain did not ask Romney for his endorsement.
Democratic candidates Hillary Clinton and Barack Obama viewed Romney as a more vulnerable candidate, preferring to run against him rather than McCain, CNN senior political correspondent Candy Crowley said.
"They were looking at Mitt Romney as pretty doable in the political sense saying, 'This is a guy that has a record that we can really run with' and they ran with it in the Republican Party as you know, saying that he used to be pro-choice, now he's anti-abortion. He has changed his position on stem cells, he has changed his position on gay unions, that sort of thing," she said.
As recently as Wednesday, Romney met with aides to discuss strategy to stay in the race through March 4.
Although he outspent his rivals, Romney received just 175 delegates on Super Tuesday, compared with at least 504 for McCain and 141 for former Arkansas Gov. Mike Huckabee, according to CNN estimates.
Romney came in first in Massachusetts, Alaska, Minnesota, Colorado and Utah on Super Tuesday. In the early voting contests, he won Nevada, Maine, Michigan and Wyoming.
After his win in the first-in-the-nation Iowa caucuses, Huckabee became Romney's chief rival for the party's conservative vote.
Huckabee on Tuesday won Arkansas, Tennessee, Georgia, Alabama and West Virginia.
Romney entered the race in early 2007 after finishing his single term as governor with the support of much of the GOP's conservative establishment.
He had campaigned as the viable conservative alternative to McCain, who has infuriated much of the party's activist base over the years.
But voters "just didn't get a sense of him that gave enough of them enough confidence," said former education secretary and radio talk-show host Bill Bennett.
"A lot of people couldn't get comfortable with Romney for one reason or another -- changes in position; 'Why is he going this way? He's a businessman, why does he sound like he is a born-again Christian?' People were just not sure of what the message was," Bennett said.
The 60-year-old former investment banker touted his management credentials throughout the campaign, citing his experience in Massachusetts and his turnaround of the scandal-plagued 2002 Winter Olympics in Salt Lake City, Utah.
Despite pouring millions of his own fortune into the campaign, Romney struggled after Huckabee upset him in the Iowa caucuses and McCain came from behind to beat him in New Hampshire.
"Primaries are a killing field," said CNN senior political analyst Bill Schneider. "They take losing candidates and get their bodies off the field."
Suspending a campaign has a different meaning depending on the party.
On the Republican side, decisions on how to allocate delegates are left to the state parties.
On the Democratic side, a candidate who "suspends" is technically still a candidate, so he or she keeps both district and statewide delegates won through primaries and caucuses. Superdelegates are always free to support any candidate at any time, whether the candidate drops out, suspends or stays in.
National party rules say a candidate who "drops out" keeps any district-level delegates he or she has won so far but loses any statewide delegates he or she has won. E-mail to a friend
CNN's Robert Yoon, Dana Bash and chief national correspondent John King contributed to this report.