By Laura Smith-Spark
BBC News, Washington
For the Mitt Romney supporters waving hastily made "Republicans Against McCain" signs, there was no question of changing their allegiance in the US presidential race.
Some are determined to prevent John McCain running for president
Standing in the lobby of the hotel where Mitt Romney had announced his decision to drop out shortly before, they predicted "disaster" if John McCain became the Republican presidential nominee.
"We are going to work like mad to make sure this candidate does not get the presidency," said Bob Shoemaker, from Virginia.
"I would vote for Hillary Clinton before I'd vote for McCain."
His big concern was Mr McCain's policy on immigration, he said, warning that it would result in tens of millions more immigrants flooding into the US over the coming decades.
Mr McCain's own mention of illegal immigration a short time later was met by boos and jeers from the audience at the Conservative Political Action Conference, suggesting many still harbour serious doubts on that score.
However, Mr McCain's plea for conservatives to focus on their shared values rather than their differences may yet pay off.
While not rejoicing at the prospect of backing John McCain, a lack of realistic alternatives gave them little choice, many conservatives at the Washington DC conference said.
Nancy Sabater, a 37-year-old businesswoman from Maryland who still clutched a thunderstick emblazoned with Mr Romney's name, said she planned to keep it "until he runs again in four years' time".
In the meantime, she said: "I'm disappointed but I will support McCain because Romney asked us to.
"I feel it's going to have to do for now - and we definitely need someone to go against Obama and Clinton.
"It's going to be difficult... I'm trying to be a positive thinker, and so I'm going to say that he can win."
Air force veteran OP Ditch will try to rethink his objections to Mr McCain
Oliver Ditch, known as OP and a retired air force colonel, said it had been "a bit of a shocker" when Mr Romney announced he was dropping out - and that some in the auditorium had been moved to tears.
But the 70-year-old - who was serving in Vietnam at the time Mr McCain was a prisoner of war there - said he would try to back the Arizona senator once he had had time to adjust.
"I have so many negatives in my mind about McCain that I will have to rationalise in my soul... I'm a true Republican at heart.
"How much I do for McCain will depend on my soul-searching."
He also articulated what many felt - that with Mr Romney dropping out of the race and the departure last month of former Tennessee Fred Thompson, he had no other choice.
"There's Ron Paul and Mike Huckabee but I don't think they are viable candidates, so I'm sure it will be McCain."
Mr McCain's efforts to extend an olive branch to the conservatives whose conference he snubbed last year may go some way to winning over the undecided, if not those violently opposed to his candidacy.
In his speech, he stressed his support for core issues such the pro-life movement and permanent tax cuts, as well as President George W Bush's troop surge in Iraq.
Nathan Hitchen, a 22-year-old intern at a Washington DC think-tank, said: "Watching McCain, he's starting to win me over.
"He definitely has a way to inspire people - his background... and he is a Vietnam veteran.
"I think he did a very good job in recognising that there were differences. He didn't come here assuming we would support him because Romney had dropped out."
Adam Brickley arrived undecided but was won over by Mr McCain
In the months to come, Mr McCain will still have to overcome the criticisms of influential conservative voices such as Rush Limbaugh and Ann Coulter, who accuse him of being liberal.
Chief among their concerns are his support for a failed bipartisan immigration bill last year that would have allowed illegal immigrants a path to citizenship and his previous opposition to the Bush tax cuts.
However, as the general election battle to win against either Hillary Clinton or Barack Obama approaches, it seems likely that most Republican voters will opt to "hold their noses" - as Mr McCain's own mother put it - and back him as the least worst option.
Adam Brickley, a 21-year-old student from Colorado who walked into the conference an undecided voter and left wearing a McCain T-shirt, agreed: "I think it's close but McCain gives us a very good chance to win.
"Most Republicans, like me, are pragmatic and will realise that he is more conservative than anything else.
"There are a few people running round here with buttons saying 'I won't vote for McCain' - but I think the vast majority of conservatives will turn out for him."