If Paul Ryan were a liberal, conservatives would describe him as a creature of Washington who has spent virtually all of his professional life as a congressional aide, a staffer at an ideological think tank, and, finally, as a member of Congress. In the right's shorthand: he never met a payroll.

If they were in a sunny mood, these conservatives would readily concede that Ryan is a nice guy who's fun to talk to. But they'd also insist that he is an impractical ideologue. He holds an almost entirely theoretical view of the world defined by big ideas that never touch the ground and devotes little energy to considering how his proposed budgets might affect the lives of people he's never met.

In making Ryan his running mate, Mitt Romney guaranteed that this election will be about big principles, but he also underscored a little-noted transformation in American politics: Liberals and conservatives have switched sides on the matter of which camp constitutes the party of theory and which is the party of practice. Americans usually reject the party of theory, which is what conservatism has now become.

In the late 1960s and '70s, liberals ran into trouble because they were easily mocked as impractical ideologues with excessive confidence in their own moral righteousness. They were accused of ignoring the law of unintended consequences and of failing to look carefully at who would be helped and who'd be hurt by their grand schemes.

Since I'm a liberal, I'd note that these criticisms were not always fair. Many of the liberals' enduring achievements -- from civil rights to environmental laws to Medicare -- grew from the boldness their confidence inspired. But, yes, there was arrogance in liberalism's refusal to take conservatism seriously.

Conservatives, in the meantime, gained ground by asking tough and practical questions: Will this program work as promised? Does it bear any connection to how the world really works? And, by the way, who benefits?

Now, it is liberals who question conservative master plans and point to the costs of conservative dreams. And in Ryan and his budget proposals, they have been gifted with a perfect foil.

How can Ryan justify his Medicaid cuts when, as the nonpartisan Kaiser Family Foundation found (.pdf), they would likely leave 14 million to 19 million poor people without health coverage? How can he justify tax proposals that, as The New Republic's Alec MacGillis pointed out, would reduce the rate on Mitt Romney's rather substantial income to less than 1 percent? How can he claim his budgets are anti-deficit measures when, as The Washington Post's Matt Miller has noted, his tax cuts would add trillions to the debt and we wouldn't be in balance until somewhere around 2030?

For Ryan, such questions (and many others arise) are beside the point because his purposes are so much grander. "Only by taking responsibility for oneself, to the greatest extent possible, can one ever be free," he wrote in the introduction to his "A Roadmap for America's Future" in 2010, "and only a free person can make responsible choices -- between right and wrong, saving and spending, giving or taking."

This is close to the definition of freedom offered by Ayn Rand, Ryan's onetime philosophical hero, in her book The Virtue of Selfishness. Ryan didn't quote Rand, but as the New Yorker's Ryan Lizza observed, he did cite a lot of intellectuals, including Milton Friedman, Adam Smith, Max Weber, Émile Durkheim, and Georges-Eugène Sorel. Didn't conservatives once dismiss this sort of thing as "a term paper"?

None of this takes away from Ryan's charm or seriousness. My one extended experience with him -- seven years ago, I moderated a thoughtful and exceptionally civil discussion about politics between Ryan and his liberal Wisconsin colleague Tammy Baldwin -- brought home to me why Ryan is so personally popular. He is great to engage with and really believes what he says.

But the issue in this election will be how Americans want to be governed. Republicans mock President Obama for still thinking like the professor he once was, yet in this race, Obama -- far more than today's conservative theorists and to the occasional consternation of his more liberal supporters -- is the pragmatist. He's talking about messy trade-offs: between taxes and spending, government and the private sector, dreams and the facts on the ground. In embracing Ryan, Romney has tied himself to the world of high conservative ideology. As liberals learned long ago, ideology usually loses.

 (c) 2012, Washington Post Writers Group