Barack Obama should use his deficit-reduction speech on Wednesday in the pursuit of one major goal: Attacking Paul Ryan’s budget and the Republicans who support it on its biggest weakness, i.e., its reliance on phony numbers. His main point should be: Democrats are the grown-ups in town, and if Republicans want to be taken seriously, they’ll have to start behaving like adults, too.

He should insist that Republicans offer real budgets with real numbers: Actual details on taxes and spending, and projections from neutral sources, not friendly think tanks. And he should demand that Republicans pass the debt ceiling hike quickly and quietly.

The truth is that the budget debate is topsy-turvy. Professional Washingtonians are giving Serious Points to Ryan for “taking on entitlements,” but give little or no credit to Barack Obama and the Democrats for passing the Affordable Care Act which not only reforms Medicare (you might remember that Republicans ran on their opposition to Medicare cuts in 2010 before apparently flipping on this in Ryan’s budget), but also deals with long-term cost control. Ryan and the Republicans oppose those cost controls, and replace them with…well, nothing. Just dumping medical costs from the federal budget (where they are a serious strain on the government) to the private market (where they are a serious strain on the economy, not to mention the millions of individuals affected). Moreover, for thirty years now it’s the Democrats who have actually acted to reduce deficits, while Republicans have talked a lot about fiscal conservativism but when they have had the chance to act, they generally just cut taxes and produce large deficits.

Obama can’t turn that around in one speech, although he certainly could remind budget idealists that health reform was in fact a major deficit reduction plan. He also can’t do much to affect public opinion; the research on presidential speeches clearly shows that their effects are mostly small and short-term. So while I understand the point that Greg made earlier today, I don’t think Obama will get much mileage at this point from talking tough on Medicare (as opposed to holding tough on it when the real negotiations begin).

What Obama can do, however, is insist that any budget proposal must be based on real numbers. It must be scored by CBO and the Joint Committee on Taxation, and it just isn’t serious — or Serious — if its projections are supplied by a friendly think tank.

The reputation of Ryan’s budget, and of Ryan and Republican deficit-cutters in general, is very vulnerable to this attack line. David Gergen types are very reluctant to hold one party responsible for budget deficits, but they really do know that Republicans have a record (going back to David Stockman and Ronald Reagan) of using phony numbers and then running huge deficits.

So Obama should press hard on the idea that there’s only one person right now with a budget based on real numbers and detailed plans, including cost-cutting on health care: Himself. Once Republicans pass the debt ceiling and come up with a real budget, then he’ll be willing to talk to them about it. Will that message change public opinion? No, because presidents can’t do that in most cases. But Obama may have a real chance to turn the Beltway elites against Ryan’s plan now by insisting that if Ryan wants to keep his Serious Points, he’s going to have to play by Serious Rules. And that means real numbers.