Aides to George W.Bush, former Reagan White House staff and friends of John McCain have all told The Sunday Telegraph that they not only expect to lose on November 4, but also believe that Mr Obama is poised to win a crushing mandate.
They believe he will be powerful enough to remake the American political landscape with even more ease than Ronald Reagan did in 1980.
The prospect of an electoral rout has unleashed a bitter bout of recriminations both within the McCain campaign and the wider conservative movement, over who is to blame and what should be done to salvage the party's future.
Mr McCain is now facing calls for him to sacrifice his own dwindling White House hopes and focus on saving vulnerable Republican Senate seats which are up for grabs on the same day.
Their fear is that Democrat candidates riding on Mr Obama's popularity may win the nine extra seats they need in the Senate to give them unfettered power in Congress.
If the Democrat majority in the Senate is big enough - at least 60 seats to 40 - the Republicans will be unable to block legislation by use of a traditional filibuster - talking until legislation runs out of time. No president has had the support of such a majority since Jimmy Carter won the 1976 election. President Reagan achieved his political transformation partly through the power of his personality.
David Frum, a former Bush speechwriter, told The Sunday Telegraph that Republicans should now concentrate all their fire on "the need for balanced government".
"It's hard to see a turnaround in the White House race," he said. "This could look like an ideological as well as a party victory if we're not careful. It could be 1980 in reverse.
"With this huge new role for federal government in the economy, the possibility for mischief making is very, very great. One man should not have a monopoly of political and financial power. That's very dangerous."
In North Carolina, where Senator Elizabeth Dole seems set to loose, Republicans are running adverts that appear to take an Obama victory for granted, warning that the Democrat will have a "blank cheque" if her rival Kay Hagen wins. "These liberals want complete control of government in a time of crisis," the narrator says. "All branches of Government. No checks and balances."
Democrats lead in eight of the 12 competitive Senate races and need just nine gains to reach their target of 60. Even Mitch McConnell, the leader of Senate Republicans, is at risk in Kentucky, normally a rock solid red state.
A private memo on the likely result of the congressional elections, leaked to Politico, has the Republicans losing 37 seats.
Ed Rollins, who masterminded Ronald Reagan's second victory in 1984, said the election is already over and predicted: "This is going to turn into a landslide."
A former White House official who still advises President Bush told The Sunday Telegraph: "McCain hasn't won independents, nor has he inspired the base. It's the worst of all worlds. He is dragging everyone else down with him. He needs to deploy people and money to salvage what we can in Congress."
The prospect of defeat has unleashed what insiders describe as an "every man for himself" culture within the McCain campaign, with aides in a "circular firing squad" as blame is assigned.
More profoundly, it sparked the first salvoes in a Republican civil war with echoes of Tory infighting during their years in the political wilderness.
One wing believes the party has to emulate David Cameron, by adapting the issues to fight on and the positions they hold, while the other believes that a back to basics approach will reconnect with heartland voters and ensure success. Modernisers fear that would leave Republicans marginalised, like the Tories were during the Iain Duncan Smith years, condemning them to opposition for a decade.
Mr Frum argues that just as America is changing, so the Republican Party must adapt its economic message and find more to say about healthcare and the environment if it is to survive.
He said: "I don't know that there's a lot of realism in the Republican Party. We have an economic message that is largely irrelevant to most people.
"Cutting personal tax rates is not the answer to everything. The Bush years were largely prosperous but while national income was up the numbers for most individuals were not. Republicans find that a hard fact to process."
Other Republicans have jumped ship completely. Ken Adelman, a Pentagon adviser on the Iraq war, Matthew Dowd, who was Mr Bush's chief re-election strategist, and Scott McClellan, Mr Bush's former press secretary, have all endorsed Mr Obama.
But the real bile has been saved for those conservatives who have balked at the selection of Sarah Palin.
In addition to Mr Frum, who thinks her not ready to be president, Peggy Noonan, Ronald Reagan's greatest speechwriter and a columnist with the Wall Street Journal, condemned Mr McCain's running mate as a "symptom and expression of a new vulgarisation of American politics." Conservative columnist David Brooks called her a "fatal cancer to the Republican Party".
The backlash that ensued last week revealed the fault lines of the coming civil war.
Rush Limbaugh, the doyen of right wing talk radio hosts, denounced Noonan, Brooks and Frum. Neconservative writer Charles Krauthammer condemned "the rush of wet-fingered conservatives leaping to Barack Obama", while fellow columnist Tony Blankley said that instead of collaborating in heralding Mr Obama's arrival they should be fighting "in a struggle to the political death for the soul of the country".
During the primaries the Democratic Party was bitterly divided between Barack Obama's "latte liberals" and Hillary Clinton's heartland supporters, but now the same cultural division threatens to tear the Republican Party apart.
Jim Nuzzo, a White House aide to the first President Bush, dismissed Mrs Palin's critics as "cocktail party conservatives" who "give aid and comfort to the enemy".
He told The Sunday Telegraph: "There's going to be a bloodbath. A lot of people are going to be excommunicated. David Brooks and David Frum and Peggy Noonan are dead people in the Republican Party. The litmus test will be: where did you stand on Palin?"
Mr Frum thinks that Mrs Palin's brand of cultural conservatism appeals only to a dwindling number of voters.
He said: "She emerges from this election as the probable frontrunner for the 2012 nomination. Her supporters vastly outnumber her critics. But it will be extremely difficult for her to win the presidency."
Mr Nuzzo, who believes this election is not a re-run of the 1980 Reagan revolution but of 1976, when an ageing Gerald Ford lost a close contest and then ceded the leadership of the Republican Party to Mr Reagan.He said: "Win or lose, there is a ready made conservative candidate waiting in the wings. Sarah Palin is not the new Iain Duncan Smith, she is the new Ronald Reagan." On the accuracy of that judgment, perhaps, rests the future of the Republican Party