Once the lights go down at the Xcel Energy Center in St. Paul after the final night of the Republican National Convention, hundreds of delegates and their hangers-on will head across town for an event that could foment as much party unity as the speech just delivered by John McCain.

At “A Tribute to the Reagan Revolution: A Late Night ’80s Celebration,” a cover band will offer hits from the Ronald Reagan decade. Michael Reagan, the president’s older son, is expected to attend.

And so clearly the party — sponsored by the American Conservative Union Foundation at the trendy Minneapolis nightclub Aqua — is meant to evoke nostalgia. But for a growing chorus of restive conservative critics, it could just as easily double as a wake for a Reagan coalition now in profound disarray.

Forecasts of the coalition’s demise have been made before, of course. But the rise of McCain to the top of the GOP hierarchy seems to be triggering more soul searching than ever among the Reaganite faithful.

While the Arizona senator offers both major blocs of the coalition something they want — fiscal conservatism for the business wing, orthodox stands against abortion rights and same-sex marriage for social conservatives — McCain is nevertheless finding his post-primary push for party unity to be an exceptionally tall order. And the reason may ultimately have more to do with the states of the principal partners in the Reagan coalition than with the broader acceptability of the candidate to the conservative movement.

Which is not by any means to say that many orthodox conservatives are enthusiastic about the presumed GOP nominee. They “fear McCain as president would destroy what’s left of the Republican brand and would finish off the conservative movement,” said Richard Viguerie, the political direct-mail mogul who was an architect of the religious right’s initial surge into power when Reagan defeated President Jimmy Carter in 1980.

Viguerie is far from the only conservative Cassandra. Radio host Rush Limbaugh has predicted that McCain in the White House “would destroy the Republican Party.” Ed Rollins, the former Reagan political director and presidential campaign manager this year for former Gov. Mike Huckabee of Arkansas, told The New York Times last year that, in his calculation, the Reagan coalition “doesn’t mean a whole lot to people anymore.”

Of course, McCain, like all the Republicans who ran for president this year, sought to lay claim to the mantle of the most revered GOP president since at least Theodore Roosevelt, describing himself as a “loyal foot soldier in the Reagan revolution.” As he starts his election campaign against Barack Obama, McCain has taken pains to compare himself favorably with the 40th president, especially on holding the line on government spending. One major television advertisement produced for the fall campaign, which began airing this month, features a photograph of McCain with Reagan.

But unlike Reagan, McCain has never managed to solidify the key business and social conservative blocs at the core of the GOP base, movement leaders say. George W. Bush succeeded in that task eight years ago, when he defeated McCain for the GOP nomination. This year, McCain prevailed in part because both halves of the coalition had a different first choice: The social conservatives preferred Huckabee, and the business community preferred former Gov. Mitt Romney of Massachusetts.

While capitalizing on that split, McCain worked during the primaries to emphasize aspects of himself with appeal to independents and centrist Democrats. It was a calculation, his conservative critics say, based largely on his assumption that the Democrats would nominate Sen. Hillary Rodham Clinton of New York, who’s so deeply loathed by movement conservatives that they would have swallowed hard and embraced McCain for the sake of her defeat.

But Obama doesn’t generate nearly as much visceral disdain. As a result, “the obvious message we’re all picking up is that the core components of the Reagan coalition don’t feel very energized,” says Dan Stein, president of the Federation for American Immigration Reform, which opposes McCain’s proposals to allow illegal immigrants to earn citizenship.

If Stein is right, it will mark a seismic shift in American politics, according to Sean Wilentz, a Princeton University history professor and liberal commentator whose new book, “The Age of Reagan,” was published this May. Wilentz credits Reagan with launching “the most sustained conservative political era in American history,” but he also agrees with those who view the Reagan coalition as on its way toward dissolving with this election.

Unlike Reagan’s successor, the first President George Bush, Wilentz says, the current president has turned out to be a dyed-in-the-wool Reaganite. But the current Bush’s enthusiasm for Reagan’s aggressive foreign policy style and his resumption of Reagan’s faith in cutting taxes as an economic curative have produced an “exhaustion of Reaganism.” That exhaustion has taken on a fatal cast, Wilentz argues, as the Iraq War has ground on and the economy has faltered.

There has nonetheless been sufficient energy in the coalition to produce infighting this year. Matt Welch, the editor of Reason Magazine, says the GOP primaries left each leg of the Reagan coalition — business interests, religious conservatives and Welch’s libertarian readers — “kicking viciously at the others in a contest for the party’s soul.” In Welch’s view, disaffection seems strongest now in the small but influential libertarian wing. One indicator was the runaway fundraising success of the libertarians’ favorite, Rep. Ron Paul of Texas, who raised more than $35 million and at times led McCain in the dollar chase.

That’s also a big reason why, when the Libertarian Party had its nominating convention in May, it was able to woo its highest-profile nominee in years, former Republican Rep. Bob Barr of Georgia. Barr is presently at about 3 percent in national polls, raising the prospect that he could throw a few close states, such as Virginia and perhaps Georgia, to Obama.

Still, as the business wing of the party ponders a Democratic administration, McCain is gaining some financial ground on his opponent. In April, a Wall Street Journal review of political contributions by people who work in seven business sectors that normally prefer GOP presidential candidates — financial services, health care, defense, energy, agriculture, construction and transportation — found McCain trailing Obama or Clinton in all of them except transportation. Since then, McCain has closed the gap, trailing Obama in only two: financial services and health care.

There’s also been a slight pro-McCain thaw in the ranks of religious conservatives. Many vividly remember the attacks McCain mounted on the religious right in 2000 but are coming to believe that he’s with them now on many of their top issues.

This month, for example, Family Research Council President Tony Perkins sent a message to supporters deriding Obama’s efforts to woo religious voters. “There is no doubt that Sen. Obama is polished on the issue of faith, but it’s his record — not his rhetoric — that speaks louder.”

But many still see such rapprochements as adding up to less than the sum of their parts — especially since McCain’s newly marshaled conservative backers are continuing to snipe at one another even as they rally behind his banner. Perkins, for example, has suggested that as president, McCain name a “family czar” as a way of reassuring social conservatives. The idea, he argued, was right out of the Reagan playbook: Reagan gave a similar job to Gary Bauer, a one-time Family Research Council president. But Dick Armey, the former House GOP leader who now chairs the conservative group FreedomWorks, scorns the proposal: “What’s happened with the evangelical wing of the coalition now is that they have missed the point that conservatism is about defending against the growth of government.”

All the infighting may be hurting the cohesiveness of groups long considered Republican locks, including opponents of illegal immigration such as Stein and gun-rights advocates.

The National Rifle Association (NRA), for example, has struggled to convince members that McCain is the candidate most supportive of gun rights. After the NRA sent out an e-mail last month attacking Obama, the group was surprised by the backlash it received. “Amazingly, some people still don’t believe Obama is radically anti-gun,” a follow-up message sent the next week said. “Some have gone so far as to claim that NRA was actually misrepresenting Obama’s anti-gun positions.”

McCain has tried — at some expense to his support among independents — to keep the coalition intact. He’s come out in favor of extending the Bush tax cuts indefinitely. He spoke in May at the NRA’s convention and, as Perkins has noted positively, announced his support for amending the California Constitution to prohibit same-sex marriage there.

At the same time, he has hedged his bets, reaching out to constituencies that were never in the Reagan coalition. He has asked Carly Fiorina, the former chairwoman of Hewlett-Packard Co., to reach out to disaffected Clinton donors and last week addressed Hispanic members of the National Council of La Raza. And those feints, as much as anything, point up the broader distress in GOP ranks that will probably linger long past the present election cycle: McCain’s campaign clearly realizes that he needs to win over enough new recruits to replace disaffected Reaganites.

FOR FURTHER READING: Independent voters, CQ Weekly, p. 1604; McCain, p. 354; evangelicals, 2007 CQ Weekly, p. 2664.