After years of upbraiding and even threatening to abolish the Internal Revenue Service, Republicans must now depend on the agency to carry out their signature legislative accomplishment: a comprehensive revision of the tax code.

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.

The House and Senate were expected to vote on the $1.5 trillion tax overhaul as soon as Tuesday. Vice President Mike Pence planned to preside over the vote and, if needed, break a tie and pass the bill.