U.S. Sen. Michael Bennet and four Democratic colleagues on Wednesday urged President Biden to keep the expanded child tax credit as part of a deal on the Democrats’ massive spending package that has been stalled in the Senate.

In a letter to Biden and Vice President Kamala Harris signed by Bennet and Democratic U.S. Sens. Sherrod Brown of Ohio, Cory Booker of New Jersey, Raphael Warnock of Georgia and Ron Wyden of Oregon, the lawmakers called the beefed-up tax credit “a signature policy achievement of this administration” and “an overwhelming success. 

They warned that dropping the measure could plunge millions of children back into poverty.

“The consequences of failing to extend the CTC expansion are dire, particularly as families face another wave of the COVID-19 pandemic,” the senators wrote. “After historic progress, it is unacceptable to return to a status quo in which children are America’s poorest residents and child poverty costs our nation more than $1 trillion per year. Raising taxes on working families is the last thing we should do during a pandemic.”

Biden suggested last week during a press conference that a one-year extension of the beefed-up child tax credit could be dropped from a revised version of the $1.75 trillion Build Back Better domestic spending package. The administration and lawmakers have been working to win the necessary support from the Senate’s 50 Democrats after West Virginia’s U.S. Sen. Joe Manchin put the breaks on the bill in December.

The expanded credit, enacted last spring as part of a $1.9 trillion pandemic-relief bill for the 2021 tax year, boosted the existing $2,000-per-child tax credit to $3,600 for children under 6 and $3,000 for children through age 17. The credit was also made fully refundable, making it available to parents who don’t earn enough to pay federal income taxes. Instead of begin available in a lump sum at tax time, monthly checks of $250 to $300 per child went out to 35 million families starting in July, with the final payments sent in December.

Bennet and his fellow senators cited estimates by economists that the program cuts child poverty by 40%, with an estimated 3.7 million children kept above the poverty line by the payments in December and.

“The Biden Administration’s expanded CTC represents the biggest tax cut for low- and middle-income families in modern American history – standing in sharp contrast to the $8 trillion in tax cuts Congress has enacted since 2001, the benefits of which have largely gone to the wealthy,” the senators wrote.

The senators also argued that the program’s benefits “go far beyond helping American families make ends meet and raise their children today,” pointing to a study released this week that found regular monthly income has a profound and far-reaching effect on infant brain development.

“Economists estimate that every dollar invested in this policy returns an additional $7 in benefits to society in the long run by improving health, education, future earnings, and other outcomes,” the senators wrote.

“Groundbreaking research released this week shows that income from monthly payments like the CTC directly affect brain development, with infants in low-income families who received 12 months of payments more likely to show brain activity patterns that have been associated with the development of thinking and learning than those who did not.”