The law has already created a new workload for the IRS - from training employees in the new system, to updating 131 federal filing systems, to answering an explosion of phone calls from taxpayers hoping for guidance.
Treasury Department officials said on Thursday that they did not expect significant changes in the number of taxpayers who receive tax refunds.
The new system ushered in by the tax reform law will be ready for 2019, but the immediate guidelines are created to allow wage earners to get immediate benefits this year without snags.
USA Today reported that last year's budget for the IRS budget was $11.2 billion, but that Congress still has not set funding levels for federal agencies, even though the current fiscal year began on October 1.
A similar impact is expected from the Republican tax law.
Employers should begin using the new withholding tables as soon as possible, but not later than February 15, according to the IRS notice issued Thursday.
The head of the National Treasury Employees Union, Tony Reardon, said the advocate's report shows the implications of years of budget cuts. A senior IRS official said he expected that level to fall just a bit next year to around 73 percent.
Though the tax overhaul was originally touted as a way to simplify the code, in some cases Congress added new complications.
Starting next month, you should see a bump in your paycheck. Yet the IRS does not have access to dates of mortgage closings.
"The IRS will have its hands full in implementing the new law", said Nina E. Olson, the taxpayer advocate, according to prepared remarks.
For the most part, the new tax law affects income earned in 2018, and the returns taxpayers must file this year deal with income earned in 2017, under the old tax rules.
Olson said funding for the IRS has dropped by about 20%, in inflation-adjusted terms, since 2010.
But that does not mean the IRS has a year to get up to speed. Employers will be required to implement these new guidelines-which instruct how much of an employee's salary is held back-by February 15, at which point the take-home pay people receive in their paychecks should change.
Republicans have said their tax law will streamline and simplify the USA tax code, in part by increasing the number of Americans who claim the standard deduction on their income taxes.
She notes that there has already been considerable public confusion about withholding changes, the deductibility of prepaid property taxes, and whether states may let taxpayers make charitable contributions in lieu of taxes to claim larger deductions than otherwise would be allowed now that there is a new $10,000 cap on the state and local tax deduction.
Trump has said the new withholding amounts could start in February.