WASHINGTON If the nation goes over the “fiscal cliff,” some Americans will wake up Tuesday with financial headaches to rival a New Year’s Eve hangover.

More than 2 million long-term jobless would receive their final unemployment benefit check within days. Millions of taxpayers would be unable to file their returns early, resulting in delayed refunds. Taxes would rise immediately on workers across the board. And although some of those increases may eventually be reversed, the first paychecks of the year would be smaller until any legislative fixes kick in.

Even if the crisis is resolved quickly after the new year as pressure mounts on President Barack Obama and lawmakers, it poses a short-term administrative nightmare for businesses. And it would be a financial blow to millions of people struggling to make ends meet in the aftermath of the recession.

As the White House and Congress try to avoid the large tax increases and federal spending cuts coming next week, taxpayers, businesses and even the Internal Revenue Service are scrambling to figure out the effects if an agreement is not reached.

The fiscal pain could be averted by a last-minute deal. And even if there is none by Tuesday, Washington policymakers could retroactively reduce tax rates if they ultimately make a deal. But the uncertainty and short-term loss of income could damage an already fragile economy.

Some effects:

• Income taxes: Rates would rise on everyone as the George W. Bush-era tax cuts expire. Middle-income households would get hit hard, paying about $1,500 more a year in taxes.

• Payroll taxes: Rates would increase by 2 percentage points with the lapse of a temporary, two-year tax cut designed to boost the economy. Workers making $50,000 annually would take home about $40 less every two weeks.

• Long-term unemployment benefits: Checks would abruptly end for people receiving extra federal aid. State benefits would still be available, but workers would be out of luck once those run out.

• Alternative minimum tax: The number of people facing the provision would skyrocket to about 33 million next year from 4 million this year. The tax, enacted in 1969, was designed to make sure the very wealthy paid some income tax. But it was not indexed to inflation and needs to be fixed each year to avoid ensnaring middle-income households.

Although Congress at some point is expected to spare most of those people from that tax, delays in doing so mean that as many as 100 million people might not be able to file their returns until the end of March or later, according to the IRS. Delays would come as the IRS has to reprogram its system, as well as for taxpayers who would have to do special calculations to determine whether they owe money because of the tax.

Businesses already are struggling to adjust. They’ve got to figure out how much in federal taxes to withhold from employee paychecks starting next week. But as of Thursday, the IRS still had not told employers what the 2013 withholding levels would be.

Workers might not see the new income tax rates immediately reflected in their paychecks. The American Payroll Association is advising its members to continue to use 2012 withholding tables until they hear differently from the IRS.

Adding to the uncertainty is the ability of Treasury Secretary Timothy F. Geithner, who oversees the IRS, to maintain the 2012 withholding levels even after tax rates rise.

Geithner could use the authority to prevent a short-term hit to the economy and potential confusion to employers if it appears a deal to prevent some of the tax rate increases was still possible even after the New Year’s Eve deadline. There’s precedent for such a move. In 1992, President George H.W. Bush ordered withholding levels temporarily set lower than federal tax rates to stimulate the economy.

Freezing withholding levels would help for a short time, because it would prevent the loss to consumers of about $10 billion every two-week pay period in higher taxes, said Mark Zandi, chief economist at Moody’s Analytics.

But it is not a long-term solution to the hit the economy would take from a continued standoff.