The Republican party is in trouble. That’s contested by some conservative pundits, but the evidence keeps mounting. Nonpartisan election-watcher Charlie Cook predicts a Democratic takeover of the House in November; last week Robert Novak called Republican midterm prospects “pitiful” and could not list a single Democratic House seat as a likely Republican pickup. Unless something changes drastically, Republicans will lose the House, lose ground in the Senate, and watch the remainder of President Bush’s presidency unravel.

If that happens, the Republican crackup which Ryan Sager predicts in his thoughtful book, “The Elephant in the Room: Evangelicals, Libertarians, and the Battle to Control the Republican Party” (John Wiley, 248 pages, $25.95), will have shown itself at the polls for the first time. Mr. Sager argues that the rift between the GOP’s two main constituent groups — social-religious conservatives and libertarians — is growing too wide to bridge and will split the coalition unless there is a change in direction.

Since the key problem for the Republican party this fall will be its own turnout, these midterm elections are a useful test for Mr. Sager’s thesis. Will the base continue to vote for GOP politicians after five years of big spending, big government, and protracted war in Iraq? What for? What’s the difference from Democrats? These will be the driving questions this fall.

If libertarians show up, then the schism would not seem nearly as acute as Mr. Sager argues. The tension has always been present, as Mr. Sager notes, quoting a wonderful bit from Abraham Lincoln on the Republican Party of 1858: “It was composed of strange, discordant and even hostile elements.” “Fusionism,” the Cold War-era answer to all this disorder, lasted for about four decades.

Popularized by a National Review senior editor, Frank Meyer, in the 1950s, “fusionism” was the alliance between social conservatives and libertarians around a small-government, strong-defense consensus. The aims of traditionalist conservatives and freedom-minded libertarians were complementary, Mr. Meyer argued, in the face of a big-government Democratic Party at home and a communist foe.

“Fusionism” served the party well. It sustained the GOP through the Goldwater years, was strained by a spendy and aggressive President Nixon, re-established itself in the Reagan years, and inspired much of Newt Gingrich’s 1994 Republican Revolution.

In the age of President Bush’s “big government conservatism,” however, the yin and yang came unbalanced. The reasons for this are many, but they start with the GOP takeover of the South. Social and religious conservatives concentrated in the southern states won increasing attention as the GOP conquered the region; meanwhile, as Mr. Sager expertly shows, libertarian-minded Republicans, who are concentrated in the interior West, were in many respects taken for granted.

The rise of Democratic governors and legislators in previously rock solid Republican interior West states is just one sign Mr. Sager identifies of the party’s neglect — made more troublesome by the projected rapid growth of the region in the coming decades. Mr. Sager is obviously partial to the libertarian strains of the party; he seems abhorrent of its Rick Santorum wing and regards the party’s overtures to “values” issues as little more than pandering. Still, Mr. Sager is careful not to let that get the best of him and does not let the thesis stray further than political reality permits — for instance, in not putting too much stock in the rise of “libertarian Democrats” in the West who think they can grab disaffected Republicans with a “leave-me-alone,” populist-sounding anti-Washington message.

What’s Mr. Sager’s solution to the coming divorce? “[T]he conservative marriage can be saved,” he writes, “but it will require something like a renewal of vows,” a renewal of fusionism: “a recognition that a limited federal government serves the interests of libertarians and social conservatives alike; a recognition that libertarian means still ultimately serve traditionalist ends.” Fair enough, and a crack-up this November would make party leaders think often and hard about it.

The two-party system has a way of forcing the type of reconciliation Mr. Sager seeks — of forging a party of the “strange, discordant and even hostile elements” of President Lincoln’s time and since. Of course, if Republicans don’t see it coming, there’s a more disastrous scenario in the wings. Imagine a third-party candidate for disaffected “leave-me=alone”-ers, a 2008 Ralph Nader — or Ross Perot — equivalent to challenge the Republican nominee. That spells a Hillary Clinton administration — which, one knows, would be more than enough to force some “fusionism” again.

Mr. Conway is an editorial writer at the Washington Times and a 2006 Phillips Foundation journalism fellow. He last wrote for these pages about Senator Clinton.