Continued from page 1

On taxes, Mr. Ryan’s plan reduces the six existing income-tax brackets to two: a 10 percent rate and a 25 percent rate. He also repeals the Alternative Minimum Tax, which was designed to eliminate tax breaks for wealthier Americans.

He would leave it up to other congressional committees to come up with a way to make sure those tax rates produce enough revenue to hit his targets.

Over the long run, Mr. Ryan’s budget would hold revenues between 18 percent and 19 percent of gross domestic product, which is in line with the historical average since World War II.

His chief changes come on the spending side, which has averaged about 20 percent to 21 percent of GDP in the postwar era. Mr. Ryan’s budget calls for lowering that to less than 16 percent by 2050, excluding interest payments on the debt, according to the Congressional Budget Office.

By comparison, Mr. Obama’s budget would have spending above the postwar average in every year.

White House Communications Director Dan Pfeiffer said Mr. Ryan’s budget “fails the test of balance, fairness and shared responsibility.”

• Sean Lengell contributed to this article.