BOSTON, Massachusetts (CNN) -- Former Republican presidential hopeful Mitt Romney announced Thursday that he is backing Sen. John McCain in his bid for the Oval Office.

"I am honored today to give my full support to Sen. McCain's candidacy for the presidency of the United States," Romney said in a joint news conference with Sen. McCain by his side.

"This is a man capable of leading our country in this dangerous hour."

Romney said he had no doubt McCain should be the next president of the United States.

After his announcement, Romney introduced McCain as a true American hero. Watch Romney endorse McCain »

McCain is CNN's Larry King's guest for the entire hour tonight at 9 p.m. ET.

CNN learned the negotiations that led to the Romney endorsement included a prominent role by John Weaver, who was McCain's top political strategist until he was forced out in a campaign shakeup last summer.

According to several sources familiar with the talks, Weaver got involved with the blessing -- indeed at the urging -- of two current top McCain advisers, Charlie Black and Steve Schmidt.

Both were aware of Weaver's friendship and past working relationship with Romney campaign manager Beth Myers. Weaver and Myers worked together on a Texas campaign and at the Texas Republican party back in the 1980s and have remained good friends

According to sources in both the Romney and McCain camps, Weaver made contact with Myers last week to initiate talks aimed at winning Romney's endorsement and an urging from him that his delegates rally to McCain's candidacy.

A source involved in the internal Romney deliberations confirmed Weaver's role and said the former Massachusetts governor concluded it is time for the party to unite behind McCain and focus on a difficult fall election environment.

One source said Romney wants to endorse McCain "in the interest of healing."

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The source said Romney also wants to help McCain move faster to "secure the nomination and unite the party for the general election against the Democrats for November."

Romney will "release" his delegates to McCain, meaning he will encourage them to get behind McCain's candidacy, the source said.

Romney had collected 286 delegates before he suspended his campaign two weeks ago.

Those delegates would give McCain 1,113 total delegates, 78 short of securing the nomination.

Romney last week suspended his campaign, saying it was in his party's best interest.

Continuing his campaign for the White House would "forestall the launch of a national campaign and be making it easier for Sen. [Hillary] Clinton or [Barack] Obama to win."

"In this time of war, I simply cannot let my campaign be a part of aiding a surrender to terror," Romney said, taking a shot at Democrats' stance on the war on terror. "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 announced he was suspending his campaign at the Conservative Political Action Conference in Washington. The crowd there booed when Romney mentioned McCain.

"I disagree with Sen. McCain on a number of issues," Romney said. "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." Watch why Romney is backing McCain »

According to a senior McCain adviser, McCain called Romney after the announcement and told him he "admired his speech today and that he was a tough competitor."

McCain's rival for the nomination, former Arkansas Gov. Mike Huckabee, presently has 217 delegates, leaving him hundreds behind McCain, according to CNN estimates.

Some Republicans worry that the longer Huckabee stays in the race, the harder he makes it for McCain to make amends with skeptical conservatives.

Huckabee gave his reaction to CNN's Wolf Blitzer by phone after the announcement. There are "still a lot of Republicans around this country who have yet to vote, many of them who feel like their voices still ... [need] to be heard." Huckabee said. "I owe it the people who got me here."

Huckabee, a former Baptist minister, has vowed to stay in until a candidate gets the 1,191 delegates needed to seal the nomination.

Romney made his announcement of the endorsement of the senator from Arizona at a news conference from Romney's Boston, Massachusetts. E-mail to a friend

CNN's Dana Bash contributed to this report.

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