From left, Rep. Richard E. Neal (D-Mass.), the ranking Democrat on the House Ways and Means Committee; Sen. Ron Wyden (D-Ore.), the ranking Democrat on the Senate Finance Committee; Senate Minority Leader Charles E. Schumer (D-N.Y.); and House Minority Leader Nancy Pelosi (D-Calif.) talk about the Republican tax plan during a news conference on Capitol Hill on Nov. 2. (AP Photo/J. Scott Applewhite)

A proposed change to the Republican tax bill that would have expanded benefits for low-income families failed on the Senate floor late Friday night.

The amendment failed by a 29 to 71 margin, despite garnering support from 20 Republicans and nine Democrats.

Sens. Marco Rubio (R-Fla.) and Mike Lee (R-Utah) proposed an amendment to the GOP's massive tax overhaul that would have given more poor families increased access to the Child Tax Credit. The Rubio-Lee amendment would have also raised the corporate tax rate in the Republican bill slightly, from 20 percent to almost 21 percent. Rubio and Lee offered Senate Republican leadership several different possibilities to pay for their $86.9 billion proposal.

Speculation mounted before the vote that Senate Democrats might band together and pass the amendment, on the theory that it moved the GOP tax bill closer to long-held progressive policy goals. But the vast majority of both caucuses wound up rejecting the proposal, which was changing rapidly throughout Friday shortly before its last-minute introduction. Democrats argued it provided only marginal relief to the poor, and the GOP was reluctant to budge on cutting corporate tax rates to 20 percent.

"With this amendment, Senators Rubio and Lee fall short of meaningful relief for millions of vulnerable American families," Sen. Ron Wyden (D-Ore.) said in a speech on the floor urging Senate Democrats to reject the expansion. "Democrats want to provide strong, permanent protection for all working families rather than temporary protection for some ... the best way to protect these families is not through a puny band-aid approach."

There were several reasons Democrats refused to rescue the Rubio-Lee amendment, many of them cited in a memo circulating in Senate Democratic offices Friday evening. Originally, Rubio and Lee had proposed tying the growth of the tax credit to inflation, which would have made its benefits more generous over time. But that provision was eventually dropped to drive down its price tag. (The Rubio-Lee amendment would still have expanded the child tax credit beyond the unamended version of Senate bill, which also does not have this proposed expansion.)

Rubio and Lee would have also required recipients of the tax credit to have Social Security Numbers, in effect excluding undocumented immigrants from receiving it. Wyden criticized this provision in his speech slamming the amendment -- though it, too, was in the existing GOP Senate proposal.

Democrats also argued that Republican leadership would have merely stripped the additional credit from the bill in the conference negotiations between the House and the Senate that are expected to start soon to resolve differences in the two chambers' bills. "McConnell was never going to allow Rubio’s provision to be in the final bill, even if it had the votes," said Rachel McCleery, a spokesperson for Wyden. "No amendment can fix this bill and we have no intention of giving Rubio an artificial win ... on an inherently partisan bill that will ultimately raise taxes on 87 million middle class Americans."

Still, some poverty experts had urged Senate Democrats to support a policy they thought would help people, even if they regarded the tax bill overall as fundamentally flawed.

“Because the tax bill overall is so deeply problematic and such ill-advised policy, if the passage of this amendment were likely to make passage of the overall bill more likely, then Democrats should vote against it,” said Robert Greenstein, president of the Center on Budget and Policy Priorities.

“However, if the bill has the votes to pass anyway, as Republican leaders now say, then in my view people should vote for the amendment because it at least makes things modestly better for millions of children and working-poor families. Which is a good thing to do,” Greenstein said.

In a floor speech earlier in the evening, Rubio said he recognized that Senate Democrats wanted the tax credit to do more. He said he also wanted the tax credit to expand beyond what his amendment proposed, but was trying to craft an amendment that could garner bipartisan support.

"I am trying to figure out, in this constitutional republic, how we can make things better, even if we do not make them perfect," Rubio said. "If we're not willing to do something as small as this, we're not willing to do anything for working people in this country. And that's an enormous problem."

Rubio and Lee were widely expected to vote for the tax bill Sunday morning, even without the inclusion of their amendment.