I found myself on TV last night arguing once again for a budget deal that balanced spending cuts and new revenues. Only later did it occur to me that I had neglected to say why that’s so important. Maybe it’s obvious, but my experience is that in debates like this with two starkly opposing views, there’s a danger of staking out your position without rationale, and that weakens your argument.

The case for revenues is a simple one. Given the level of deficit reduction to which both parties are committed, around $4 trillion over 10-12 years, if you try to get there solely by cutting spending, you’ll be forced to cut too deeply into parts of government that vulnerable people depend on. In short, you’ll do more harm than good.

Specifics? Rep Ryan’s budget, embraced by House Republicans, is a good example. As my CBPP colleague Bob Greenstein shows here, two-thirds of its spending cuts, almost $3 trillion, come from programs that help low-income families:

“$2.17 trillion in reductions from Medicaid and related health care; $350 billion in cuts in mandatory programs serving low-income Americans (other than Medicaid [so things like food assistance, child care credits, unemployment benefits]; $400 billion in cuts in low-income discretionary programs.”

More specifics come from this important editorial in today’s NYT, which points out that exempting low-income programs from deficit-cutting deals “has been a major feature of deficit deals going back to 1985.” That tradition is especially germane today, with the economic recovery still far from reaching the most vulnerable among us.

All this from a plan that delivers $700 billion of permanently extended Bush-era tax cuts to the wealthy. If you dropped in from outer space and looked at the R’s budget, you would conclude that the big economic problem facing our nation is that poor people have too much income and rich people have too little.

And that, my friends, is why we need a plan that doesn’t depend solely on spending cuts.