Republicans boasted all last year that their new tax law boosted paychecks and showered bonuses on several million workers.

But now that tax season is upon us, several million Americans are getting a nasty surprise : a bill from the Internal Revenue Service that they never expected.

Beth Callori of Long Island, New York, said she was thrilled to receive about $90 more in each paycheck last year. Thanks to the new lower federal income tax rates, Calloris employer, a financial services firm, was withholding less from her paycheck for federal tax purposes.

I thought, Wow, Trump is great, I love him, Callori said.

But last week Callori heard from her tax preparer that she owes the federal government more than $5,000 almost five times as much as she had to pay in previous years.

I almost fell out of my chair. I could not believe it, she said. I voted for Trump. I thought he was going to be good for this country, but when I got that phone call, thats it, Im done.

Calloris tax bill went up for two reasons. One is that the law directly disadvantaged her by limiting deductions for state and local taxes, which increased the amount of Calloris income subject to tax and added an extra grand to her bill.

The bigger reason is that her employer withheld too little from her paycheck. The extra $90 she received should have been added to the amount that gets automatically socked away to cover the federal income tax. Like most people, however, Callori did not fill out a worksheet and submit a new Form W-4 to her employer at the beginning of last year.

After all, at that time Republicans kept bragging about the bigger paychecks they had given the American people.

I thought because I was getting that, Im entitled to it, Callori said.

Bigger Paychecks, Lower Refunds

The vast majority of Americans got lower taxes from the new law, while only 5 percent or so should have seen a tax increase. Most people should have seen the changes in their paychecks last February.

But the way the Trump administration implemented the law has caused a separate problem one that the administration knew would result in something like 5 million fewer households receiving tax refunds this year. Its still early in tax filing season, which opened at the end of January, but the average refund is down 8.7 percent so far.

The Treasury Department suggested tax refunds are bad anyway because they result from people overpaying the government.

Smaller refunds mean that people are withholding appropriately based on their tax liability, which is positive news for taxpayers, a spokesperson said in an email.

The problem is, its not just smaller refunds its that paycheck withholding for this tax season is less accurate in general.

Treasury has said it expected the percentage of people withholding too much tax in their paychecks to decline from 76 to 73 percent, but the percentage withholding accurately is not increasing at all. Instead, Treasury expected the rate of under-withholding to go from 18 to 21 percent. Those people all owe the IRS.

The households most at risk are ones with higher incomes, two earners and slightly more complicated taxes especially households that used to itemize their deductions. Instead of taking the standard deduction, which reduces taxable income by a set value, itemizers would add up what they spent on state and local taxes, mortgage interest and charitable giving, and deduct that sum instead. The Tax Cuts and Jobs Act greatly reduced the advantage of itemizing and limited the deduction for state and local tax.

Politics Over Planning

But the Trump administration decided not to make major changes to the withholding tables that employers are required to use to make sure everybody is paying the right amount of tax. The value of allowances that workers can choose on Form W-4 to adjust their withholding has been pegged for years to something called the personal exemption which the new tax law eliminated. So they set the value of allowances to last years personal exemption and adjusted for inflation.

Coming up with new forms would have taken at least half a year, and would have been a chore for everyone.

Theres this tension Youd like to get all the information needed to calculate withholding as accurately as possible, but that gets very complicated for taxpayers, said Joe Rosenberg, a researcher at the Tax Policy Center.

The administration could have let people withhold too much rather than surprise them with bills at tax time, Democrats have said.

It looks like the Trump Treasury Department spent 2018, an election year, goosing peoples paychecks by under-withholding, and it should have been obvious that the bill would come due eventually, Sen. Ron Wyden (D-Ore.) said in a statement Friday.

The IRS did try to warn people to check their withholding. The agency told the Government Accountability Office that it put out press releases and sent emails to listservs with hundreds of thousands of subscribers. It promoted a paycheck checkup campaign on Twitter and Facebook, and officials talked to the media.

The make sure youre not underwithheld messaging may have been drowned out by congressional Republicans bragging that their law had turbocharged the economy and directly benefited millions of workers. They kept a running tally of the hundreds of firms that had announced bonuses for their employees, trumpeting each announcement in a series of press releases . Fox News worked overtime to remind people that House Speaker Nancy Pelosi previously called the bonuses crumbs, as though that werent a perfectly accurate description .

Another problem, which is not Republicans fault, is that even though everyone pays taxes, not everyone understands them very well. Less than half of Americans said they knew they could update their W-4 forms at any time and only 19 percent had actually done so, according to a November survey by the tax prep company H&R Block. Twice as many survey respondents said they updated their W-2, which is actually a document prepared by employers, not workers.

Also, some filers are surprised by tax bills simply because they didnt realize some of the money they earned needed to be taxed. Theyll have some side job where theres no withholding and thats creating this new tax liability, H&R Blocks Nathan Rigney said, pointing to the rise of non-employee gigs like Uber and Taskrabbit.

Crunching Numbers

By capping deductions for state and local taxes, Republicans knew the tax hikes in their law would be concentrated on states with high taxes which tend to be led by Democrats, who use the taxes to provide more social services.

But its not just wealthy New Yorkers whove wound up paying more.

Kurt Kromm is an electrician in Kenosha, Wisconsin an area that used to be represented by former House Speaker Paul Ryan (R-Wis.). Kromm knew he and his wife would pay higher taxes because of the limit on deductions for state and local taxes, but he didnt think they would owe more than $4,000 this spring.

Kromm had already indicated on the Form W-4 documents hed previously submitted to his employer that he was married and wanted zero allowances, which means he was withholding as much as he could without specifying an additional dollar amount.

I figured married and zero would probably be appropriate, Kromm said. They spent no time trying to really put out a decent withholding table.

Kromms case shows the two-earner household pitfall. After he realized how much he owes, he went to the withholding calculator on IRS.gov and realized he could have either told his employer to take an additional $80 per week or just chosen to withhold at a single rate on his W-4, since single people face higher rates and therefore higher withholding. If he had done so last year, he could have had less money in each paycheck but saved himself the aggravation of making a large payment.

But not even the administration expected people to go to such lengths in its simulations of how people would be affected, the IRS assumed nobody would adjust their W-4s.

Its unreasonable to expect working people with busy lives to start the year out by crunching the numbers on their tax withholding with the rigor of a workaholic [certified public accountant], Wyden said.

Beth Callori, for her part, said she had also previously chosen zero allowances plus she had an extra $130 withheld from each paycheck. It wasnt enough.

She said she used to love Donald Trump. Not anymore.

I really liked things he was doing, until this, and now I hate him, she said.