Paul Ryan had not finished his acceptance speech when President Obama's campaign manager, Jim Messina, released a statement blasting him for favoring "new budget-busting tax cuts for the wealthy." Ryan's "radical" budget plan, Messina added, "would end Medicare as we know it by turning it into a voucher system, shifting thousands of dollars in health care costs to seniors."

Romney could have run from this fight. Plenty of Republicans have been telling him not to distract voters from their unhappiness about the economy by presenting far-reaching plans of his own -- and especially not to elevate the issue of Medicare, on which Democrats traditionally have an advantage.

Romney has rejected this advice. As Messina's statement also shows, this campaign will now be about something big: the future of the welfare state.

The Democratic attack on Ryan and his budget plan is, at best, ignorant. "Budget-busting" tax cuts? The Ryan budget plan envisions taxes running at a higher level of GDP than the average of the last few decades.

Ryan's Medicare plan doesn't shift health care costs to seniors, either. The Democrats' criticism was arguably true of Ryan's 2011 plan. The 2012 version -- the one Romney endorsed -- includes changes to meet that criticism. That's how it won the support of Sen. Ron Wyden, D-Ore.

Under the original Ryan plan, retirees would have chosen a private health plan, and the government would have contributed toward the cost. The amount of subsidy would have depended on the beneficiary's age and health status. Over time, the average amount of money would have risen with inflation.

Critics pointed out that health care costs have long risen faster than inflation. If competition failed to change that, seniors would indeed have been left paying more.

The new version of the plan cleverly fixes the problem. Insurers would submit bids to see who could cover Medicare's traditional benefits for the lowest premium. The average amount of financial assistance would equal the second-lowest bid. So seniors will always have an option that leaves them with no higher costs than now. If they pick something even cheaper, they come out ahead.

Ryan's budget includes a fail-safe to make sure the plan saves money even if competition doesn't restrain premium growth: Total spending would be limited to the growth of the economy plus inflation plus 0.5 percent.

That fail-safe doesn't rescue the Democratic attack, however, because the Obama administration caps Medicare spending at the same level. There is no scenario under which Medicare recipients have to pay more under the Romney-Ryan plan than they have to pay under the Democratic plan.

The Ryan budget isn't perfect. The better criticism, though, is that it doesn't go far enough on entitlements, particularly Social Security. Too large a share of the budget restraint therefore comes from domestic programs other than entitlements. This isn't, unfortunately, a criticism that Democrats have any interest or credibility in making.

Beneath Messina's distortions lies a real debate. Is our welfare state basically healthy, just in need of a few tweaks? Democrats believe, or claim to believe, that if we just raised taxes on the rich and let experts redirect Medicare spending, we could keep the open-ended entitlement programs on which we have come to rely.

Republicans tend to think that our entitlement programs are structurally flawed in a way that neither tax increases nor better management can solve. Republicans do not want to abolish these entitlements. Their view is that they should be limited, and made to work with rather than against markets.

The Democratic view has a strong base of support among voters. Even those who share the Republican view, as I do, cannot be sure it will win in the court of public opinion.

If Romney and Ryan do prevail, it will mean voters accept the need to modernize the welfare state -- and this election will have been the most important one since 1980.

Ramesh Ponnuru is a senior editor at National Review. He wrote this article for Bloomberg News.