WASHINGTON — President Biden’s plan to erase significant amounts of student loan debt for tens of millions of Americans could cost about $400 billion, the nonpartisan Congressional Budget Office said in a report Monday, making it one of the costliest programs in the president’s agenda.

The C.B.O. said the price tag might rise even higher because of Mr. Biden’s decision to extend a pause on federal student loan repayments through the end of the year, which could end up costing some $20 billion. The report gauged the cost over a period of 30 years, though the bulk of the effects to the economy would be felt over the next decade.

Although the office called the figures “uncertain,” they are generally in line with those that economists put forth after Mr. Biden announced the program in August. The report is certain to revive the political debate over student loan forgiveness just weeks before the midterm elections. Critics have cast the plan as a costly giveaway that could exacerbate inflation, while the administration argues that it will help millions of low- and middle-income Americans get their footing in a volatile economy.

Mr. Biden’s plan cancels $10,000 in debt for those earning less than $125,000 per year and $20,000 for those who had received Pell grants for low-income families. In its report, the C.B.O. said that of the 37 million borrowers with direct loans from the federal government, 90 percent who are eligible could be expected to take advantage of debt forgiveness once it becomes available. (White House officials have suggested that a far smaller share of eligible borrowers are likely to opt into the program than the budget office predicts, which would reduce its cost.)

In a joint statement, Senators Chuck Schumer of New York, the majority leader, and Elizabeth Warren of Massachusetts, two Democrats who pressured Mr. Biden to enact the policy, said the C.B.O. estimate “makes clear that millions of middle class Americans have more breathing room thanks to President Biden’s historic decision to cancel student debt.”

Still, critics have accused the Biden administration of hiding the plan’s true cost.

Marc Goldwein, the senior vice president for the Committee for a Responsible Federal Budget, said that the C.B.O. score did not take into account a significant part of the administration’s loan relief program: a plan to reduce payments for future borrowers who go on to earn low incomes after college, which outside analysts say could host hundreds of billions of dollars more.

“You’re basically buying a very expensive lottery ticket,” Mr. Goldwein said. “When you’re taking out the loan, you’re going to have no idea of how much you’re going to be paying back.”