Taxpayers could take $400 off their income taxes for each child under 17. That helped families with decent incomes and several children, but did nothing for the large number of taxpayers who don’t owe any income tax — currently about 40% of households.

Starting in 2001, advocates for low-income families, led by Rep. Rosa DeLauro of Connecticut, pushed through a change to allow families to get some of the money even if they don’t owe any taxes. That’s called refundability in tax jargon, and it’s key to making the tax credit work as an antipoverty measure.

Their argument was that if the federal government was going to use the tax code to boost families with children, it should focus on those most in need.

Over the last two decades, DeLauro and others, including former Speaker Nancy Pelosi and Sens. Michael Bennet of Colorado, Sherrod Brown of Ohio and Cory Booker of New Jersey, made expanding the credit a top priority for congressional Democrats.

Step by step, they got provisions into each big tax bill that expanded the amount of the credit — now $2,000 per child — and the amount that could be refundable. In some of those efforts, Democrats struck alliances with Republicans, including Sens. Marco Rubio of Florida and Mitt Romney of Utah, who have seen the tax credit as a pro-family measure.