Although the Senate version of the bill still doubles the child tax credit from $1,000 to $2,000 annually per child, Jonathan Abbamonte, a research analyst at the Population Research Institute, said “the Rubio-Lee Amendment would have helped stem the tide of declining fertility.”

“It would have helped reduce the financial burdens for child rearing on working families,” Mr. Abbamonte said. “However, Rubio’s proposed increase to the nonrefundable tax credit per child that was included in the Senate version of the bill is better than nothing and appears likely to cause a smaller, but nonetheless important, boost to the fertility rate.”

The National Center for Health Statistics released an analysis this year showing the U.S. fertility rate fell to a record low of 62.0 births per 1,000 women of reproductive age in 2016, down from 62.5 in 2015.