Last August left little doubt that a conservative revival was underway. Constituents packed town-hall meetings across the country to confront Democratic House members and senators ill-prepared to explain why, in the teeth of a historic economic downturn and nearly 10% employment, President Obama and his party were pressing ahead with costly health-care legislation instead of reining in spending, cutting the deficit and spurring economic growth.

Still, whether that revival would have staying power was very much open to question. A year later—and notwithstanding the Democrats' steadily declining poll numbers and the mounting electoral momentum that could well produce a Republican majority in the House and a substantial swing in the Senate—it still is.

Sustaining the revival depends on the ability of GOP leaders, office-holders and candidates to harness the extraordinary upsurge of popular opposition to Mr. Obama's aggressive progressivism. Our constitutional tradition provides enduring principles that should guide them.

In late 2008 and early 2009, in the wake of Mr. Obama's meteoric ascent, the idea that conservatism would enjoy any sort of revival in the summer of 2009 would have seemed to demoralized conservatives too much to hope for. To leading lights on the left, it would have appeared absolutely outlandish.

President Ronald Reagan Associated Press

In late October 2008, New Yorker staff writer George Packer reported "the complete collapse of the four-decade project that brought conservatism to power in America." Two weeks later, the day after Mr. Obama's election, Washington Post columnist E.J. Dionne proclaimed "the end of a conservative era" that had begun with the rise of Ronald Reagan.

And in February 2009, New York Times Book Review and Week in Review editor Sam Tanenhaus, writing in The New Republic, declared that "movement conservatism is exhausted and quite possibly dead." Mr. Tanenhaus even purported to discern in the new president "the emergence of a president who seems more thoroughly steeped in the principles of Burkean conservatism than any significant thinker or political figure on the right."

Messrs. Packer, Dionne and Tanenhaus underestimated what the conservative tradition rightly emphasizes, which is the high degree of unpredictability in human affairs. They also conflated the flagging fortunes of George W. Bush's Republican Party with conservatism's popular appeal. Most importantly, they failed to grasp the imperatives that flow from conservative principles in America, and the full range of tasks connected to preserving freedom.

Progressives like to believe that conservatism's task is exclusively negative—resisting the centralizing and expansionist tendency of democratic government. And that is a large part of the conservative mission. Progressives see nothing in this but hard-hearted indifference to inequality and misfortune, but that is a misreading.

What conservatism does is ask the question avoided by progressive promises: at what expense? In the aftermath of the global economic crisis of 2008, Western liberal democracies have been increasingly forced to come to grips with their propensity to live beyond their means.

It is always the task for conservatives to insist that money does not grow on trees, that government programs must be paid for, and that promising unaffordable benefits is reckless, unjust and a long-term threat to maintaining free institutions.

But conservatives also combat government expansion and centralization because it can undermine the virtues upon which a free society depends. Big government tends to crowd out self-government—producing sluggish, selfish and small-minded citizens, depriving individuals of opportunities to manage their private lives and discouraging them from cooperating with fellow citizens to govern their neighborhoods, towns, cities and states.

Progressives are not the only ones to misunderstand the multiple dimensions of the conservative mission. Conservatives have demonstrated blind spots, too.

In 2010—in an America in which the New Deal long ago was woven into the fabric of our lives—conservatives can not reasonably devote themselves exclusively to limiting the growth of government. Government must effectively discharge the responsibilities it has had since the founding of the republic, but also those it has acquired over more than two centuries of social, political and technological change.

Those responsibilities include putting people to work and reigniting the economy—and devising alternatives to ObamaCare that will enable the federal government to cooperate with state governments and the private sector to provide affordable and decent health care.

A thoughtful conservatism in America—a prerequisite of a sustainable conservatism—must also recognize that the liberty, democracy and free markets that it seeks to conserve have destabilizing effects. For all their blessings, they breed distrust of order, virtue and tradition, all of which must be cultivated if liberty is to be well-used.

To observe this is not, as some clever progressives think, to have discovered a fatal contradiction at the heart of modern conservatism. It is, rather, to begin to recognize the complexity of the conservative task in a free society.

To be sure, the current conservative revival was not in the first instance inspired by reflection on conservative principles.

The credit for galvanizing ordinary people and placing individual freedom and limited government back on the national agenda principally belongs to President Obama, House Speaker Nancy Pelosi and Senate Majority Leader Harry Reid. Their heedless pursuit of progressive transformation reinvigorated a moribund conservative spirit, just as in 1993 and 1994 the Clintons' overreaching on health care sparked a popular uprising resulting in a Republican takeover of Congress.

The Gingrich revolution fizzled, in part because congressional Republicans mistook a popular mandate for moderation as a license to undertake radical change, and in part because they grew complacent and corrupt in the corridors of power.

Perhaps this time will be different. Our holiday from history is over. The country faces threats—crippling government expansion at home and transnational Islamic extremism—that arouse conservative instincts and concentrate the conservative mind.

Mr. Berkowitz is a senior fellow at Stanford University's Hoover Institution.