The task is monumental. While processing tens of millions of tax returns for 2017 under the current rules, the I.R.S. would also have to figure out how to interpret and explain a new system and put it into practice.

“Beware,” said John A. Koskinen, who retired last month as I.R.S. commissioner. “If the budget keeps being cut and the agency keeps being given more things to do, the I.R.S. is simply not going to work.” Either the information technology will fail, forcing the filing and refund systems to collapse, he warned, or enforcement and audits will become so scarce that fewer people will be inclined to pay the taxes they owe.

Even before Congress began revising the tax code, the I.R.S. was struggling to keep up with an expanding workload. Since 2010, its budget has been cut by $900 million — or 17 percent, after adjusting for inflation — and its staff reduced by 21,000, or 23 percent. In the meantime, it has had to process roughly 10 million more individual returns.