A $2 trillion proposal from William G. Gale and Benjamin H. Harris, via The Hamilton Project at Brookings:
We propose a value-added tax (VAT) to contribute to the U.S. fiscal solution. A 5 percent broad-based VAT, paired with subsidies to offset the regressive impacts, could raise about 1 percent of GDP, or about $160 billion, per year. Although it would be new to the United States, the VAT is in place in about 150 countries worldwide and in every non–U.S. OECD country. In recent years, the VAT has raised about 20 percent of the world’s tax revenue (Keen and Lockwood 2007).
This experience suggests that the VAT can raise substantial revenue, is administrable, and is minimally harmful to economic growth. Additionally, the VAT has at least one other potential advantage worth highlighting: a properly designed VAT might help the states deal with their own fiscal issues. Although a VAT would be regressive relative to current income, this regressivity can be easily offset by transfers that would make the net burden progressive. A VAT should only be imposed after the economy has returned to full employment, as the depressing effects of increased taxation in a demand-driven economy would suppress the economic recovery.
This would basically add a full percentage point — for starters — to federal revenue as a share of GDP. The problems with the VAT aside, I am particularly uninterested in the idea without seeing what the accompanying changes to the income tax code would look like. In general, I am more interested in complete replacement options.