In our report card grading the tax provisions of the stimulus bill, our highest mark went to the Making Work Pay credit because employers could deliver it to workers quickly by reducing withholding. The IRS, we were told, could implement new withholding tables by July so households would have more money to spend throughout the second half of the year.

President Obama wanted the credit to start even sooner and ordered the IRS to have new tables ready in time for employers to reduce workers’ withholding by April 1. The IRS must have anticipated the change—just four days after the president signed the stimulus bill into law, the IRS announced new withholding tables and posted them on their website a week later.

As expected, low-income workers will get no immediate benefit from the credit, even though it is refundable. A single worker with one dependent must earn at least $195 a week ($9,750 for the year) to see any increase in take-home pay and at least $830 a week ($43,160 a year) to get the maximum $400 credit as increased net pay. A married person with two children must earn $440 a week ($22,880 a year) to benefit at all from reduced withholding and $750 a week ($39,000 a year) to get the $16 a week maximum increase in net pay.

Wondering how the IRS deals with married couples? The stimulus act says couples can get up to $800, whether one or both spouses work. Rather than require employers to ask workers whether their spouses work, the tables split the difference, increasing net pay by up to $16 a week for married workers—a total of $624 over the 39 weeks in the year after April 1. If both spouses work enough so that each gets that full increase, their withholding would fall by $1,248—more than half again the credit they may claim on their 2009 tax return—and possibly leave them underwithheld.

And what happens for people who work two jobs or some couples for whom the credit phases out? The amount withheld again falls by more than the credit’s value, so some taxpayers may have too little withheld to cover their taxes. (The IRS does warn that “Individuals and couples with multiple jobs may want to submit revised Form W-4 forms to ensure enough withholding is held to cover the tax for the combined income” but workers may fail to do so.)

Despite the tables’ limitations—a one-size-fits-all table that is simple to use is an unattainable holy grail—the IRS gets a pat on the back for issuing them so quickly. Workers will have more to spend only a few weeks from now, just as the president intended.

Closer to home for those of us at TPC who get paid by the Urban Institute, our payroll folks are ahead of the game. They promise that we’ll see the credit in our March paychecks.

Let the spending begin.