According to the Congressional Budget Office, tax revenue will average 19.9% over the next decade, reaching 20.8% in 2021, if Congress doesn't act. That's in part because of the scheduled expiration of income tax cuts at the end of 2012. Extending those tax cuts would yield revenue averaging about 18% for the decade, according to CBO.

'Pockets of Taxpayers'

Republicans who favor holding revenue at postwar levels make several points. One is the past-is-prologue argument, the contention that the government and the economy functioned well enough during the postwar period.

"I abide by the principle that 18% of the GDP of this country is good enough for the government to spend," Senator Charles Grassley, an Iowa Republican, said in an April 13 floor speech. "That leaves 82% in the pockets of the taxpayers for them to decide how to spend, because if 535 of us decide how to divide up the resources of this country, it doesn't do as much economic good."

The period from World War II through today is fundamentally different from the coming period, because of the retirement of the baby boom generation and the continuing rise of health care costs at a rate faster than the economy, Burman said.

Without changes, Medicare and Social Security will cost about 9.3% of GDP by 2020 and 12.1% by 2035, compared with 8.4% today, according to CBO.

Holding revenue to a percentage of GDP "completely ignores the demographic changes," Burman said.

Focus On Spending Cuts

Republicans such as House Majority Leader Eric Cantor and Senator Orrin Hatch say spending cuts should be the sole path to deficit reduction.

"Washington does not have a revenue problem," Cantor said on Fox News April 14. "It's got a spending problem."

A spending-only approach, as outlined in the budget authored by Ryan and passed by the House this month, would require major changes to Medicare and Medicaid that would leave future beneficiaries with less generous health care, said Alan Viard, resident scholar at the American Enterprise Institute, a Washington policy center that favors smaller government.

"It requires changing these commitments in a way that I think is not going to happen politically," Viard said.