Democrats, faced with an effort by House Republicans to repeal the health care overhaul, have once again seized on the argument that the health care law will reduce the national deficit. And once again, they’re pointing to projections made by the Congressional Budget Office in order to make their case.

The CBO’s report is hardly surprising: Last year it said that if the law were executed exactly as planned, it would reduce the deficit. This year the congressional scorekeeper is saying that if the law were repealed, it would increase the deficit. The scores for last year’s bill also included a number of warnings and caveats, noting that the scores only estimate what will happen if the legislation remains unchanged and the proposed cuts and efficiency gains are implemented successfully. The rough score for the repeal bill includes a similar cautionary note:

As with all of CBO’s cost estimates, these estimates—both for the first 10 years and beyond—reflect an assumption that the provisions of current law would otherwise remain unchanged throughout the projection period and that the legislation being considered would be enacted and implemented in its current form. CBO’s responsibility to the Congress is to estimate the effects of proposals as written and not to forecast future legislation. However, current law now includes a number of policies that might be difficult to sustain over a long period of time. If those policies or other key aspects of the original legislation would have subsequently been modified or implemented incompletely, then the budgetary effects of repealing PPACA and the relevant provisions of the Reconciliation Act could be quite different—but CBO cannot forecast future changes in law or assume such changes in its estimates.

And remember, although the CBO does not technically forecast changes based on implementation failures or future legislation, it has nonetheless put up what amounts to a flashing neon sign indicating that it believes there is a significant chance that the law’s long-term savings won’t come through: In last summer’s alternative budget scenario—widely considered to be the more realistic scenario produced by the office and a better picture of the nation’s actual fiscal future—the CBO included the assumption that much of the law’s planned long-term savings would not pay off.

So despite the salience of deficit politics, this is not an argument that’s likely to give Democrats much traction: If you thought the CBO’s initial scores were the most likely guess about the law’s deficit effects, you’ll probably buy the argument that repealing the law will expand the deficit. But if instead you thought the CBO’s initial scores were the product of successful efforts by the bill’s Democratic backers to game the scoring process, you’ll probably remain skeptical of the Democrats’ argument that keeping the law in place is the deficit-reducing, fiscally responsible thing to do.