There’s a longing on the left for the golden years of the 1940s, ’50s and early ’60s. Income distribution was significantly more egalitarian than it is today, and Americans had far more confidence in big government, the wisdom of our elected officials, and the ability of Keynesian spending policies to stimulate economic growth.

Hence the search for policies that will somehow get us back to those golden years. The Obama Democrats have been desperately trying to increase membership in labor unions, to the point of threatening to close down Boeing’s new Dreamliner plant in South Carolina. They passed an $814 billion stimulus package and ObamaCare. And they’re still itching to raise tax rates on high earners, though they botched it when they had supermajorities in Congress.

But the America of the past is a different country to which we can’t return. As Andrew Levison recently lamented in the Nation magazine: “Doubts about the ability of government to create jobs reflect not only a disbelief in Keynesian remedies for unemployment but also the profound doubts many Americans have about government in general.”

Still, liberals pine for what I call America’s Midcentury Moment. It was the product of World War II, lasting from 1940 until the mid-1960s when the wartime experience wore off and the emerging baby boomers led culture and politics in another direction. For those of us who grew up in those years, the Midcentury Moment seemed the norm in American experience. But in fact it was the result of a unique time in U.S. history, when a united nation was mobilized for total war and Americans were, literally or figuratively, put into uniform.

It started in the months before Pearl Harbor, as President Franklin Roosevelt mobilized the nation and its economy for the war he believed necessary to eradicate the scourge of Hitler and fascism. In September 1940, he signed the bill instituting the military draft. One year later ground was broken on the Pentagon, which remains the largest office building in the world. American industrial firms were enlisted into war production. Rationing began soon after war was declared. Auto production was ended, with assembly lines turning out Jeeps and tanks and aircraft.

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NY Daily News via Getty ImagesGrand Central Station, 1946

Unions agreed not to strike in return for government encouragement of unionization and higher wages. Government spending rose to 40% of gross domestic product, financed partly by confiscatory taxes on high earners and even more by mass voluntary purchases of war bonds. Big government, big business and big labor all united in the effort to deliver on what Roosevelt promised in his speech one day after the attack on Pearl Harbor: “[T]he American people in their righteous might will win through to absolute victory.”

This massive mobilization reshaped our national mores for a generation in ways that we find hard to comprehend. At one time or another 16 million Americans served in the military. The equivalent proportion of today’s population would be 38 million Americans serving in the military over the next three and a half years—something none of us can imagine. Nor can we envision ourselves paying taxes at World War II rates, accepting rationing of butter and meat and rubber, doing without new cars, or putting most of our wage and salary increases into low-interest government bonds.

Only by keeping in mind those experiences can we fully appreciate the exhilaration that came from victory. In two words with catchy internal rhyme—”righteous might”—Roosevelt conflated the ideas that the American people were both strong and good.

Victory in World War II conferred enormous prestige on the leaders of the big units—big government, big business, big labor—who had led the war effort at home. No wonder that levels of confidence in the big units and their leaders remained high for a generation—higher, I suspect, than they had ever been before the Midcentury Moment and higher, certainly, than they have been since.

No wonder, also, that Americans in the Midcentury Moment were unusually conformist, content to be very small cogs in very large machines: They married and bore children at record rates for an advanced society; they worked as organization men and flocked to mass-produced suburbs; they worshipped in seemingly interchangeable churches. This was an America that celebrated the average, the normal, the regular.

The liberals who long to return to the Midcentury Moment seem to forget that it was a time of enormous cultural uniformity that stigmatized being unmarried or unchurched or gay. The huge menu of lifestyle choices from which we can choose today was a very short menu with very few choices then.

It could not last. Baby-boom children, raised in prosperity, were not content with being small units in large machines. The Berkeley student activists in 1964, before the major escalations in Vietnam, held signs reading, “Do not bend, staple, fold or mutilate”—I am not just another IBM card. The military draft, which more than anything else initiated the Midcentury Moment and was supposed to apply equally to everyone, was by 1965 so riddled with exceptions and loopholes that the sons of the well-to-do were largely exempt from military service in time of war. Similarly, the tax code in the early 1960s had enough exceptions and loopholes that high tax rates on high earners were eminently avoidable.

Vietnam, urban riots, Watergate, stagflation—all undermined confidence in big government, big business and big labor, and by the late 1970s the Midcentury Moment was long gone. It has not returned and it is hard to conceive of circumstances in which it could. Big labor is no longer big, except for the public-employee unions. Big business has been subject to enormous change to the point that the Fortune 500, fairly stable during the Midcentury Moment, has seen new firms enter and old ones disappear at record rates. As for big government, its prestige has never fully recovered, leaving the military as one of our few respected institutions and the civilian government largely concerned with transferring money from current earners to the elderly at rates that are economically unsustainable but politically difficult to alter.

So the Obama Democrats, partially successful in expanding the size and scope of government, largely unsuccessful in reviving private-sector unions, are on the defensive politically. As Mr. Levison and other liberals recognize, most Americans don’t accept Keynesian economics and don’t favor expansion of government as they did during the Midcentury Moment. Thus the Democrats’ 2012 campaign strategy seems aimed more at discrediting Republican alternatives than seeking endorsement of their own policies.

But there is a more fundamental contradiction here, for the Midcentury Moment’s confidence in big institutions was inextricably connected with an acceptance of a cultural uniformity that almost all of today’s liberals, and probably most non-liberals, would find unacceptable.

Mr. Barone is senior political analyst for the Washington Examiner, resident fellow at the American Enterprise Institute and co-author of “The Almanac of American Politics,” published by National Journal.