When Treasury Secretary Steven Mnuchin was asked at his confirmation hearing what he thought about using private companies to collect money owed to the government, he replied that it “seems like a very obvious thing to do.”

It may have been obvious, but it certainly was not economical.

Private debt collectors cost the Internal Revenue Service $20 million in the last fiscal year, but brought in only $6.7 million in back taxes, the agency’s taxpayer advocate reported Wednesday. That was less than 1 percent of the amount assigned for collection.

What’s more, private contractors in some cases were paid 25 percent commissions on collections that the I.R.S. made without their help, according to the annual report by Nina E. Olson, who heads the Taxpayer Advocate Service, an independent office within the I.R.S.

While Republicans have been the most vocal proponents of privatizing public services, congressional Democrats are equally responsible for the I.R.S.’s program. Despite the pointed failure of similar efforts in the past, Congress passed a law in 2015 requiring the I.R.S. to use outside contractors to make a dent in the $138 billion that taxpayers owe the government.