WRAPUP 5-Republican McCain asks conservatives for support

(Adds Obama agrees to debates, paragraph 22)

By John Whitesides, Political Correspondent

WASHINGTON, Feb 7 (Reuters) - Republican John McCain asked disgruntled conservatives to support his presidential bid on Thursday, shortly after Mitt Romney ended his struggling campaign and made McCain the all-but-certain nominee.

McCain assured a conference of conservative activists he was one of them, citing his commitment to win in Iraq, halt Iran's nuclear ambitions and rein in the federal government while drawing sharp contrasts with potential Democratic opponents Hillary Clinton or Barack Obama.

McCain's speech, which drew boos on the topic of illegal immigration, followed by a few hours Romney's surprise announcement at the conference that he was ending his run to allow Republicans to focus on the November election.

"I feel I have to now stand aside, for our party and for our country," the former Massachusetts governor told the shocked crowd, some of whom gasped and shouted "No, no!" in response.

McCain, who has built an almost insurmountable lead in delegates to the party's nominating convention, pleaded for party unity during his appearance at the annual conference.

McCain, the 71-year-old former Vietnam prisoner of war, has become a target of critics on the right for his moderate views on illegal immigration, his votes against President George W. Bush's tax cuts in 2001 and 2003 and his labeling in 2000 of some religious conservative leaders as "agents of intolerance."

"I know I have a responsibility, if I am, as I hope to be, the Republican nominee for president, to unite the party and prepare for the great contest in November," the Arizona senator told the activists gathered in a Washington hotel.