Obama and Paul Ryan both addressed the American Association of Retired Persons today, framing one of the key battles that will unfold in the election’s home stretch. Obama’s argument was straightforward: My approach to bringing down the deficit and securing the future of entitlements is balanced and won’t ask seniors to foot the bill; my opponents’ approach is unbalanced and will ask for too much sacrifice from seniors — even as it would also cut taxes deeply for the rich.

Asked what he would do secure Medicare’s future, Obama argued that the health law was already beginning to answer that question — by focusing on reducing health care costs, rather than seniors’ benefits — and contrasted that with Paul Ryan’s approach:

“The other side’s approach to saving Medicare...is to turn Medicare into a voucher program, and essentially transfer that cost onto seniors....he said, we’re gonna have traditional Medicare stand side by side with a voucher program. And no current beneficiaries will be affected. The problem is that insurance companies, once they’re getting vouchers, they’re really good at recruiting the healthier, younger Medicare recipients, and leaving out the older, sicker recipients. And over time what happens is — because there are older, sicker folks in the traditional Medicare plan, premiums start going up...And the entire infrastructure of traditional Medicare ends up collapsing. Which means that all seniors at some point end up being at the mercy of the insurance companies .”

Paul Ryan gave a speech that sounded a bit more like a political rally. He slammed Obama for failing at bipartisan outreach, told far more personal stories, and called again for repeal of Obamacare, which elicited boos. Repeal, of course, would reverse the work the law is doing in extending Medicare benefits to some seniors. Ryan’s case for his plan:

“Our plan empowers future seniors to choose the coverage that works best for them from a list of plans that required to offer at least the same level of benefits as traditional Medicare. This financial support system is designed to guarantee that seniors can always afford Medicare coverage, no exceptions. If a senior wants to choose the traditional Medicare plan, then she will have that right... we will make no changes fort hose in or near retirement. And for my generation, we can make this program solvent by slightly raising the retirement age over time and slowing the growth of benefits for those with higher incomes.”

Ryan’s political challenge is to minimize the degree to which his plan would undermine the fundamental promise at Medicare’s core, and to downplay the ideological nature of his crusade to alter entitlements, a task that got a bit tougher with revelations that he described Social Security as “collectivist” in 2005. The Romney/Ryan strategy throughout has been to obscure the true nature of their differences with Obama over how to secure Medicare’s future. But today, the basic difference was there for all to see: Obamacare is about saving the program by reducing medical costs; Ryan would reduce costs through private sector competition, he claims, but also would raise the retirement age and would reduce benefits.

Polls suggest that seniors are more supportive of Ryan and Romney than they are of Obama. But this battle isn’t just about seniors; it’s about the future retirement of all other Americans, and about the set of priorities and view of government’s proper role in guaranteeing economic security during that retirement that should guide big upcoming choices. On these subjects, there are big and fundamental differences that can’t be obscured.

Obama’s closing argument will be this: We will have to reach a series of compromises sooner or later on how to reform entitlements and fix the deficit. When fiscal crunch time hits, who do you trust to really defend your interests — me, or Mitt Romney, Paul Ryan, and a bunch of Tea Partyers in Congress?