...its [sic] odd to see smart people talking as though the set of planks that make up each party's platform are bound together in some coherent way that flows from the two timeless essences of American political thought. It seems equally true to say simply that the mix of positions held by each party is the equilibrium response to the mix adopted by the other. As these debates over party identity show, this isn't necessarily the case in the short term, but the very identity to which purists want to hew is itself necessarily the product of the harsh evolutionary pressures of the electoral system.I would submit that since its inception and with a few aberrations the Republican Party, like its Whig predecessor, has stood mainly for protecting the interests of the powerful -- minimizing taxation and regulation, maximizing the police and military power of the state, supporting religious authorities and the "traditional values" they uphold (though in one key contradiction, supporting business interests entails removing restrictions on advertising and entertainment that undermine traditional values).
"Republicanism" just means "the combination of views that were historically capable of securing a majority often enough to establish one of the two governing coalitions". Juggle the initial conditions—the demographic facts or the issues that are salient—and you almost certainly get a different coalition mix. I understand why one segment of the coalition would be eager to see their own views determine the direction of the party as a whole, but it seems silly to express this in terms of the language of authenticity.
Promoting business interests have sometimes led the Republicans (and Whigs) to support policies that today would seem more suited to the Democrats: Federal investment in infrastructure, protective tariffs -- and for a season, isolationism. But that's because business interests (real or perceived) changed, not because Republican interests did.
Viewed positively, the Republican party has sometimes stood against excesses in social engineering , taxation and government bureaucracy effected by the Democrats, and at times quite effectively protected and advanced American interests abroad. Or one might say, as Bill Gates has, that Republicans are about supporting efforts to generate wealth, and Democrats about finding ways to share it equitably.
While Democrats, since Andrew Jackson's time, have purported to represent the interests of the less powerful, the great exception is slavery and segregation. At the time the Republican Party was formed, Democrats were more sympathetic to and inclined to treat with the South, and after the War they cemented an unholy alliance with Southern segregationists. I won't pretend to know more about this than I do, but my recollection is that at the time of the Civil War, poor northern whites and their political representatives did not regard the plight of African American slaves as their problem. The Republicans, on the other hand, represented the elite in an economic system that rendered slavery impractical, and viewed slavery (quite rightly) as a hindrance to economic development. And in the case of Lincoln and others, there's a real connection between viewing human beings as free economic agents and viewing them as free, period.
Andrew Sullivan approves of the DiA sentiment, I think, because he does not like to acknowledge that "conservatism" in America -- and probably just about everywhere, in practical terms -- exists chiefly to "save" existing power structures. In a society in which the ruling classes are not purely self-interested, that's not always a bad thing. Some institutions are worth preserving. Burke tapped into a durable truth by warning of the vast unintended consequences of violent or sudden overthrows of existing order. Sometimes, too, conservatism stumbles into conserving freedoms, to the extent that they're part of the existing order or support the existing order. But the notion that conservatism is essentially about protecting liberty by limiting the size and power of government is an ideological smokescreen, at least as far as exercise of power in the real world is concerned. Conservatives want to limit the power of government to redistribute wealth and regulate those who control wealth. When in power, they rarely if ever support limiting government's power to restrict civil liberties or otherwise exercise control over individuals, other than when they're making money.