Republican candidate Mitt Romney dropped out of the US presidential race on Thursday, a decision that almost certainly will make McCain the nominee of his party.
But that is an unappealing prospect to some conservative white evangelical Protestants, whose support could prove to be the difference in the November election against a galvanised Democratic Party.
McCain is an abortion-rights foe but his failure to support a constitutional amendment to ban gay marriage and backing of embryonic stem-cell research are among the political heresies that some conservative evangelicals cannot forgive him for.
With the influential James Dobson, the founder of the conservative advocacy group Focus on the Family, already saying he will not vote for McCain, analysts say evangelical turnout - or lack thereof - could be key on November 4.
"It's possible that the lack of enthusiasm for McCain could lead to a lower turnout among evangelicals in the fall," said Scott Keeter of the Pew Research Center.
That scenario could tilt the election in favour of the Democrats as Republicans have come to rely heavily on an evangelical community energised to get out and vote by its opposition to abortion rights and gay rights.
Their vote was widely seen as the difference for President George W Bush in his two successful White House runs.
"Anything short of a fully engaged and mobilised Republican base will spell disaster for the Republican nominee," said Tony Perkins, president of the Family Research Council, a conservative lobby group with strong evangelical ties.
"Evangelicals do more than vote ... they volunteer, they work in campaigns. They'll do volunteer phone work and pass out flyers," he said.
