The selection of Paul Ryan — chairman of the House Budget Committee — as Mitt Romney’s vice presidential candidate has the potential to turn this dreary presidential campaign into a meaningful debate over the size and role of the federal government. It could also (sadly) litter the debate with so many exaggerations, distortions and falsehoods that Americans end up less informed and less able to make sensible choices.

Until now, the campaign has been a disheartening descent into a swamp of negative ads and personal character attacks that, if hardly unprecedented, seem unusual in that they’re increasingly made by the candidates themselves. Any semblance of a reasoned debate over the nation’s future has been conspicuously absent.

It can be observed: The candidates are trying to win; they’re not conducting a civics class; it has always been thus.

There never was a “mythic gold age” of campaigns, historian Gil Troy of McGill University writes in the current Wilson Quarterly.

As campaigns became more democratic, they became less informative, he says. In the 19th century, election days were already “mass carnivals, capping months of squabbling, pamphleteering, parading, speechifying, [and] mudslinging.” The “aspects of the campaigns that Americans hate reflect the democracy we love,” Troy argues.

Obama’s and Romney’s resort to personal attacks has reflected standard political logic: It beats the alternatives. Obama can’t run on his record, because his record isn’t strong. Payroll jobs remain 4.8 million below their pre-recession peak. The Affordable Care Act (Obamacare) — his singular legislative achievement — isn’t popular; it is disliked by 44 percent of the public and liked by only 38 percent in the latest Kaiser Family Foundation poll. For his part, Romney has so far failed to paint a compelling vision for the United States. He’s campaigned mostly on Obama fatigue.

The selection of Ryan potentially alters this logic.

More than anyone in Congress, Ryan is identified with a basic reappraisal of the federal government. He has proposed a sweeping budget plan for reducing projected spending. Liberals hate Ryan’s plan, arguing that it would shred the safety net and cut deeply into many programs. Conservatives love it for coming to terms with the nation’s future. Politically, both sides may welcome the debate because each thinks it has the winning argument. Obama will accuse Romney of gutting government, especially Medicare, Romney will retort that Obama ensures top-heavy government and sluggish economic growth.

A genuine debate is long overdue. The framework for government’s expansion since 1960 has broken down. Its political appeal was that we got more government (from food stamps to Medicare to Pell grants) without a parallel increase in tax burdens. In 1960, federal taxes were 17.8 percent of the economy (gross domestic product); in 2007, before the financial crisis, they were only 18.5 percent of GDP.

Two bits of good fortune enabled this something-for-nothing swelling of government: First, annual economic growth averaged slightly more than 3 percent; second, defense spending declined as a share of the budget — in effect, lower defense spending financed higher social spending. Both props are gone.

Even with a full recovery from the Great Recession, economic growth is sinking toward 2 percent annually. The main cause is the impending retirement of baby-boom workers, which will stunt growth in the labor force. As for defense, past reductions mean that present cuts don’t permit much other spending. In 1970, defense was 42 percent of federal outlays; in 2011, even with the war in Afghanistan, it was 20 percent.

Combined with the surge of baby-boomer retirees, which will increase Social Security and Medicare spending, these trends have shoved governmental accounts massively out of balance. There would be huge deficits even had there been no Great Recession. The fact that there was adds complexity, because overzealous cuts might imperil the recovery.

“America’s presidential campaign process works,” argues Troy, the historian. “It sifts through candidates, facilitates a continent-wide conversation and, most important, bestows legitimacy on the winner.”

Up to a point, this is convincing. The grueling, prolonged campaign reveals character, values, temperament and political competence — the ability to connect with people, respond to unanticipated events and exert leadership. But it’s unclear how well the present campaign will serve its most important role after anointing a victor: conferring legitimacy.

It’s impossible to close long-term budget deficits simply by taxing the rich and cutting defense (liberal dogma) or eliminating “waste” and “unneeded” spending (conservative dogma). The “conversation” conducted by Obama and Romney needs to conform to economic and budget realities. If it doesn’t, Americans will discover after the election that they’ve been had.

The question hovering over this election — and over American democracy — is whether our leaders can navigate the fundamental political change. For half a century, government bestowed benefits on masses of citizens. Now, it must revoke benefits or raise taxes. Ryan provides the question, though not the answer.