It is tax time, the season when the country’s largest antipoverty program, the earned income tax credit, plows billions of dollars into mailboxes and bank accounts of low-income working Americans like Ms. Spain. It is the most important financial moment of the year for many people in the bottom half of the wage bracket, a time to pay off old bills, make car repairs, buy children clothes and maybe make a big purchase like a refrigerator or a TV.

As incomes among the country’s lowest wage earners continue to stagnate, the credit has played a critical role in smoothing the hard edges of an unforgiving labor market for the country’s most vulnerable workers and helping stem the tide of income inequality that has been rising among Americans in recent decades.