WASHINGTON  The package of spending and tax cuts proposed by Senator John McCain is unlikely to achieve his goal of balancing the federal budget by 2013, economists and fiscal experts said Monday.

“It would be very difficult to achieve in the best of circumstances, and even more difficult under the policies that Senator McCain has proposed,” said Robert L. Bixby, executive director of the Concord Coalition, a nonpartisan budget watchdog group.

Mr. McCain, the presumptive Republican presidential nominee, is proposing billions of dollars in tax cuts. But advisers to Mr. McCain said those costs would be more than offset by savings from slower growth in spending.

In his proposal, Mr. McCain said he would hold overall spending growth to 2.4 percent a year. That is a tall order because federal spending has been growing an average of more than 6 percent a year in the last five years.