(Carolyn Kaster/AP)

That’s because Ryan is doing what Romney will not: Saying what, specifically, Republicans intend to do if they take power. As a veteran politician, Romney knows better than to get into those weeds. In a recent interview with the conservative Weekly Standard, he explained that voters who want to know which spending he’ll cut and which agencies he’ll eliminate will have to elect him to find out:

“One of the things I found in a short campaign against Ted Kennedy was that when I said, for instance, that I wanted to eliminate the Department of Education, that was used to suggest I don’t care about education,” Romney recalled. “So I think it’s important for me to point out that I anticipate that there will be departments and agencies that will either be eliminated or combined with other agencies. So for instance, I anticipate that housing vouchers will be turned over to the states rather than be administered at the federal level, and so at this point I think of the programs to be eliminated or to be returned to the states, and we’ll see what consolidation opportunities exist as a result of those program eliminations. So will there be some that get eliminated or combined? The answer is yes, but I’m not going to give you a list right now.”

Even when Romney is specific, he’s vague. Romney released a fairly detailed plan to reform Medicare into a premium-support system, but he left out the most crucial piece of information: Whether his vouchers would keep pace with the cost of medical care. Ryan’s budget included few such mysteries.

But Ryan’s budget was fulsomely endorsed by Romney. “I applaud it,” he said. “It’s an excellent piece of work, and very much needed.” And then it was passed by 228 House Republicans. Romney might wish to be vague, but he — and his party — have signed onto something quite specific. And that gives Obama something to run against.

Throughout the Republican primary, Romney’s great weakness has been his chameleon-like nature. Conservatives never really believed Romney was one of them. But that is, arguably, a strength for Romney in the general election: Independent voters might also believe that Romney’s conservatism is an election-year show, and he would govern as the reasonable moderate from Massachusetts.

Romney’s studied vagueness is an effort to make it easier for voters to take this leap: The less specific he is about what he would actually do, the more voters can project their own preferences onto him.

By lashing Romney to Ryan’s budget, Obama intends to lash him to the conservative wing of the Republican Party. Where Romney has purposefully refrained from filling in the details on his agenda, the Obama campaign intends to use Ryan’s blueprint to fill them in on Romney’s behalf.

This can only work because Romney can’t actually walk away from Ryan’s budget: He’s already called it “an excellent piece of work” and campaigned alongside Ryan. Worse, Romney knows full well that conservatives would shriek if he began publicly distancing himself from the policy framework that the Republican Party has coalesced around. And so here’s Obama:

Instead of moderating their views even slightly, the Republicans running Congress right now have doubled down, and proposed a budget so far to the right it makes the Contract with America look like the New Deal. (Laughter.) In fact, that renowned liberal, Newt Gingrich, first called the original version of the budget “radical” and said it would contribute to “right-wing social engineering.” This is coming from Newt Gingrich.

And yet, this isn’t a budget supported by some small rump group in the Republican Party. This is now the party’s governing platform. This is what they’re running on. One of my potential opponents, Governor Romney, has said that he hoped a similar version of this plan from last year would be introduced as a bill on day one of his presidency. He said that he’s “very supportive” of this new budget, and he even called it “marvelous” — which is a word you don’t often hear when it comes to describing a budget.

It’s a bit unusual for the president of the United States to give a major address about a specific policy proposal from the other party, if only because the president’s prestige elevates the other party’s proposal in the mind of the public. But, in this case, that’s exactly what the White House is hoping will happen.

Obama is trying to increase the visibility of Ryan’s budget. He’s attacking it precisely because that will make Republicans rally around it. He’s trying to make everyone agree that Ryan’s budget is the Republican agenda. And, ultimately, he’s trying to make Romney answer for it. If the White House has its way, they’ll spend the rest of this year campaigning against Paul Ryan even as they run against Mitt Romney.