Salena Zito

Former Massachusetts Gov. Mitt Romney ended his Republican presidential bid Thursday, acknowledging that he didn't want to split the party by helping the political opposition during the war on terror, calling it the campaign's key issue.

Romney, 60, a favorite among many conservatives partly because of his business acumen, said he would continue to fight for the principles that shaped his campaign, but that the greatest challenge facing "the entire civilized world (is) the threat of violent, radical Jihad" and he could not fight on to the Republican National Convention because "we are a nation at war."

His withdrawal from the race, during a speech to the Conservative Political Action Conference in Washington, all but secures the nomination for Sen. John McCain of Arizona.

McCain needs 1,191 delegates to win at September's convention in St. Paul, Minn. He won 707 delegates in primaries and caucuses through Super Tuesday, compared to Romney's 294 and 195 for former Arkansas Gov. Mike Huckabee, according to a tally by The Associated Press.

Romney gave a tepid endorsement of McCain in his speech, saying he disagrees with McCain on a number of issues but "I agree with him on doing whatever it takes to be successful in Iraq ... and on eliminating al-Qaida and terror."

Continuing a contested GOP race could make it more likely that Democrat Sens. Hillary Clinton or Barack Obama might win, he said, and "they would retreat and declare defeat" in Iraq.

"In this time of war, I simply cannot let my campaign be a part of aiding a surrender to terror," Romney said, acknowledging the decision to quit was difficult. "If this were only about me, I would go on. But I entered this race because I love America, and because I love America I feel I must now stand aside, for our party and for our country."

Romney said Americans must fight to preserve the culture of hard work, education and opportunity that made the United States the most powerful nation in history. The country's prosperity, security and future are threatened, he said.

"I am convinced that unless America changes course, we will become the France of the 21st century -- still a great nation, but no longer the leader of the world, no longer the superpower," he said. "And to me, that is unthinkable."

McCain, 71, promised to uphold conservative principles in a speech that followed Romney's to a skeptical audience at the conference.

"I am proud to be a conservative," he said. "You need not examine only my past votes and speeches to assure yourselves that they are my genuine convictions. You can take added confidence from the positions I have defended during this campaign."

Salena Zito

Salena Zito is a political analyst, reporter and columnist.