"People ask me all the time how we delivered four surplus budgets," former President Bill Clinton said on Wednesday night. "What new ideas did we bring? I always give a one-word answer: arithmetic."

Jonathan Newton/The Washington Post

That's also the one-word answer to what Clinton brought to his convention speech. To a degree unusual in political rhetoric, this was a 48-minute speech about arithmetic. About math. About budgets.

It was, in a sense, the speech that I expected Paul Ryan to give last week. Not in its particulars, of course. It would have been rather off-message if Ryan had mounted the stage and said, "I want Barack Obama to be the next President of the United States." But in its tone. In its specifics. In its wonkiness. Bill Clinton made the policy case for Barack Obama. Neither Paul Ryan nor anyone else at the Republican convention made a sustained policy case for Mitt Romney.

In that way, Clinton's speech fit neatly into the emergent Democratic strategy to be, in this election, the party of policy. To be sure, they don't have much of a choice. The difference between the Democratic and Republican tickets right now is the Democrats are stuck with thousands of pages of policy while the Republicans have made a strategic decision to avoid having much policy at all.

Romney has omitted the core details of his tax, spending, housing, financial regulation, and health-care plans for a reason. He's made a point of distancing himself from core elements of the Ryan budget -- like the Medicare cuts -- for a reason. His campaign made a judgment that they had a better chance of winning this election if they made it about Obama's policies rather than Romney's.

The Obama campaign, conversely, can't escape their policies. The Affordable Care Act is out there. The Dodd-Frank financial regulations are out there. The stimulus is out there. There's nothing really to do but defend them, and to try and create a contrast with the other side.

That's what Democrats, with their focus on the Affordable Care Act, tried to do last night. And it's what Clinton, in a considerably more comprehensive manner, did tonight, mounting a thorough defense of Obama's record, offering an unusually detailed rebuttal to many of the Romney-Ryan ticket's attacks, and opening some new fronts in the policy debate. The most of important of these was his commentary on Medicaid.

Now, folks, this is serious, because it gets worse. And you won’t be laughing when I finish telling you this. They also want to block grant Medicaid and cut it by a third over the coming 10 years. Of course, that’s going to really hurt a lot of poor kids.

But that’s not all. A lot of folks don’t know it, but nearly two-thirds of Medicaid is spent on nursing home care for Medicare seniors who are eligible for Medicaid. It’s going to end Medicare as we know it. And a lot of that money is also spent to help people with disabilities, including a lot of middle-class families whose kids have Down’s syndrome or autism or other severe conditions.

And, honestly, just think about it. If that happens, I don’t know what those families are going to do. So I know what I’m going to do: I’m going to do everything I can to see that it doesn’t happen. We can’t let it happen. We can’t.

A quick note: I think, but don't know for sure, that when Clinton said it will "end Medicare as we know it," he meant it will "end Medicaid as we know it." I'd check the transcript against Clinton's prepared remarks, but he treated his text as little more than a reminder of which subject he wanted to riff on next, and the line in question was part of his improv.

That said, there are three things worth noting about this Medicaid section. First, it's a direct attack on Romney and Ryan's claim that their budget won't harm any seniors over age 55. As Clinton notes, much of Medicaid's spending goes to nursing home care for seniors, and there's no way Romney and Ryan can cut the program by a third without hurting the seniors who account for the plurality of Medicaid's spending.

Second, it's an inarguable attack on Romney and Ryan's budgets. There are many, many places where one or both of the Republican candidates have been vague about they actually intend to cut or change, and that's made it hard for Democrats to mount a sustained assault on them. But both Romney and Ryan have been very clear about how, and by how much, they intend to cut Medicaid. They will block grant it, and cap its growth such that it gets about $1.4 trillion less than it would under Obama. It's a rare instance of specificity in both of their budgets, and so it can't be waved away by promising details that will come after the election.

Third, it's arguably the most important and concrete policy difference between the two campaigns. The Medicare changes get more attention on both sides, but Romney and Ryan don't intend to touch Medicare for 10 years, they swear they'll honor the Medicare guarantee, and at least in Ryan's most recent budget, he envisions the exact same long-term spending path as Obama does. By contrast, Romney and Ryan intend to begin cutting Medicaid immediately, and independent analyses suggest that their cuts could throw as many as 30 million people off the program. If you want to see the difference between Obama and Romney's vision for American policy, it's probably the single starkest example.

Which is, no doubt, why Clinton chose to highlight it. Tonight, his role was wonk-in-chief, and he was trying to persuade the public of an old idea: That the best way to understand this election is to simply do the arithmetic. And so, for 48 minutes, that's pretty much what he did. The question now is whether the Romney campaign can persuade voters that there's a mistake in Clinton's math.


Read more: The full transcript of Clinton's speech.