WASHINGTON – Sens. Marco Rubio and Mike Lee should use leverage to force negotiators to include their proposal to increase the child tax credit, a prominent group said Wednesday.

"During the Senate debate, the Senate rejected the Rubio-Lee amendment, but now that the bill is in a conference committee with the House, the only route for Senators Rubio and Lee is to use the leverage of their votes to secure this change — as Senator Johnson did to secure a larger tax cut for wealthy investors. Otherwise, Republicans will cement their choice to pass a $1.5 trillion tax bill without providing virtually any meaningful relief to the lowest-income working families," wrote Chuck Marr, director of federal tax policy for the left-leaning Center on Budget and Policy Priorities.

"Adopting the Rubio-Lee amendment wouldn't fix either the Republican tax bill's extreme tilt to the top or its fiscal profligacy, nor would it change the fact that key congressional Republicans are already saying they will seek to pay for their tax cuts next year by cutting programs that support low-income working families. But this modest change would allow millions of low- and moderate-income working families to fare somewhat better."

Rubio and Lee proposed setting the corporate tax rate at 22 percent, not 20 percent, to make for a more generous child tax credit for moderate and lower income families. They later changed it to 20.9 percent. But the Senate rejected the plan.

Rubio still voted for the overall deal, even though he could have taken a more forceful stand.

We have asked Rubio's office for comment.

Even under the Rubio-Lee proposal, low-income working families would see far smaller CTC increases than families at higher income levels. But unlike the current bill, the Rubio-Lee proposal offers many of these 26 million children a more meaningful increase in their CTC. Starting with their first dollar of earnings, parents would qualify for a CTC of 15.3 cents per dollar of earnings — enough to offset the payroll taxes that workers face on their wages (including both the employee and employer shares).  Consider just a couple of examples:

A single mother with two children who works full time at the federal minimum wage as a home health aide would receive a $494 CTC increase under the Rubio-Lee proposal, or $247 per child. That's far less than the Senate bill's CTC increase of $1,000 per child but more meaningful than the $75 she'd receive under the Senate bill (and the $0 she'd get under the House bill).

A married couple with two children and earning $24,000 would receive a $1,672 CTC increase under the Rubio proposal — close to the Senate's full increase of $1,000 per child and far better than the $200 they'd get under the House and Senate bills.