Sens. Marco Rubio and Mike Lee failed late Friday night to expand the child tax credit for low-income families in the tax bill as it moved toward passage, as the vast majority of Democrats and Republicans voted to rule their amendment out of order.

The two Republicans had offered language to expand the bill’s $2,000 child tax credit to low-income families by making it refundable against payroll tax liability. The amendment failed 29-71, with 20 Republicans voting in favor.

Rubio, R-Fla., and Lee, R-Utah, had not said publicly how they would vote on the underlying bill if their amendment failed.

Their proposed change would have extended the benefit of the credit to millions of families. But they would have offset the lost revenues by setting the corporate tax rate at 20.94 percent, rather than the 20 percent called for in the bill. That transfer, which Rubio said would amount to about $87 billion over 10 years, meant that the amendment drew opposition from some business and conservative groups that favored lower corporate tax rates.

The amendment vote provided a test of the influence of “reform conservatives,” outside advisers and analysts who have advocated for the GOP to focus more on family-friendly policies.

It also proved a trickier vote for Democrats, who had otherwise found it easy throughout the past few days to stridently oppose the tax cut bill. Some crossed the aisle to support the measure.

In a speech on the Senate floor, Rubio defended the amendment as a way to ease the struggles faced by working families adjusting to the changing economy and claimed that slightly increasing the corporate tax rate wouldn’t undermine the Republican effort to boost economic growth.

“You’re telling me that if we have a corporate tax rate that goes from 35 percent to 20.94 percent — that’s gonna hurt growth?” he asked rhetorically. “Twenty percent is the most phenomenal thing we’ve ever done for growth, but if you add .94 percent to that, it’s a catastrophe?”

“If we’re not even willing to do something as small as this, we’re not willing to anything for working people in this country,” he said.

Democrats argued that the provision didn't go far enough in expanding the credit.

"With this amendment, Sens. Rubio and Lee stop far short of meaningful relief for millions of vulnerable American families, and leave out altogether so many deserving children, like the Dreamers," said Ron Wyden of Oregon.

Sen. Sherrod Brown, D-Ohio, a proponent of significantly increasing the child tax credit, acknowledged, however, that the two Republicans have made a “real effort” to gain bipartisan support for a bigger credit.

Rubio’s fellow Florida senator, Democrat Bill Nelson, also expressed support for the amendment on the Senate floor earlier in the evening, saying that “we should be coming together in a bipartisan manner" to trade a slightly higher corporate rate for a more expansive child tax credit. Nelson was one of nine mostly moderate Democrats who voted for the amendment.

Republicans, meanwhile, faced conflicting counsel on the amendment from outside groups.

On Twitter, Americans for Tax Reform head Grover Norquist warned that the amendment will “begin the climb back to 35%” on the corporate rate and said it “must be stopped." The Koch network of political groups also opposed the amendment.

The proposal had support, however, from some conservative intellectuals and was endorsed by National Review.

April Ponnuru, a senior advisor at the Conservative Reform Network, applauded the Senate for increasing the tax credit in the first place. "Senators Rubio and Lee deserve enormous credit for fighting to allow working families to keep even more of their hard-earned money, and I'm disappointed that a majority of Senators didn't see fit to support that effort," she said. "Nevertheless, progress has been made, and I have every confidence that we can build on it."