James Carter and Jason Fichtner describe it here:
Any viable solution must cut spending growth. Sen. Mike Enzi of Wyoming and Rep. Connie Mack of Florida have introduced legislation in their respective chambers to do just that. Their “Penny Plan” – recently updated to reflect the latest budget developments – calls for reducing federal spending (excluding interest payments) 1 percent a year for five years, balancing the budget in the fifth year.
To maintain balance once it’s reached, Mr. Enzi and Mr. Mack would cap federal spending at 18 percent of GDP. By no small coincidence, 18 percent of GDP roughly matches the U.S. long-run average level of taxation since World War II.
They report that the plan currently has 71 House supporters and 12 Senators.