The policy, written by the Department of Education, is under review by the White House budget office and has drawn fierce opposition from Republican lawmakers and school administrators.
It was first proposed in September, but the department appears to have rushed to adopt it since Donald Trump won the presidential election. Adopting the regulation before Friday at noon would bind the incoming Trump administration unless Congress overturns it.
Republicans on Capitol Hill have pledged to oppose many "midnight regulations" from the Obama White House, and the issue could also come up at the nomination hearing Tuesday for Donald Trump's secretary of education, Betsy DeVos.
The regulation would dramatically increase federal control over spending at many of the nation's 14,000 school districts and force some districts to increase spending at low-income schools.
Districts could achieve the increase by diverting money -- a total of $800 million nationwide -- from more-affluent schools, or by spending an additional $2.2 billion in state and local funds, the Education Department says.
The regulation would apply only to school districts that have both low-income schools that get Title I federal funds and higher-income schools that are not eligible for the money.
"If this were to be finalized, it would be a huge deal because it's saying how local and state money inside school districts is to be distributed across schools," said Nora Gordon, a school-finance expert at Georgetown University.
Scott Sargrad, a former Education Department official, said the regulation "could significantly improve achievement for students" in low-income schools.
"This is the federal government explicitly saying poor students need to have at least the same resources as their higher-income counterparts. That's a significant step for the federal government to say that," said Sargrad, who is now an education policy expert at Center for American Progress, a liberal Washington think tank.
But opponents including Sen. Lamar Alexander, R-Tennessee, say federal law does not give the administration the authority to enact such a far-reaching regulation.
Alexander, chairman of the Senate education committee and a former secretary of education, has vowed to block the regulation -- a move that would require a majority vote in both houses of Congress and the President's signature.
Congress could also effectively stymie the regulation by denying the department funding to implement it.
The National School Boards Association called the regulation "unnecessary" and "unwarranted federal overreach" that would constrain school districts.
After accepting public comments for 60 days, until early November, the department took just four weeks to send a final version of the regulation to the White House budget office for review.
Departments and agencies typically take at least several months -- and sometimes years -- to write a final rule after receiving public comments.
"They've been pushing this rule pretty hard," said Nat Malkus, an education-policy expert at the conservative American Enterprise Institute.
The budget office has given no indication of when and if it will approve the regulation. The Education Department has not released its final version, although experts expect it will closely resemble the initial proposal.
Department spokeswoman Dorie Nolt would say only that the agency "is still working on the final rule."
Students at low-income schools shortchanged
Education Secretary John B. King Jr. has championed the regulation as a way of improving education for low-income students by requiring school districts spend the same amount of state and local dollars at their low-income schools as they do at higher-income schools.
An Education Department study in 2011 found many low-income schools were receiving less in per-pupil funding than their higher-income counterparts in the same district.
The department says that 3.3 million of the nation's 50 million public-school students attend low-income schools that receive less money per pupil than higher-income schools in the same district. As a result, the department says, nearly 6,000 low-income schools "are shortchanged by about $440,000 a year."
In announcing the proposal in August, King called it "an important step forward to advancing resource equity across the country." King also urged districts and states to comply with the regulation "by providing additional funds for education focused on high-needs schools, not by shifting dollars around or forcing transfers of teachers or other personnel."
The regulation interprets a small section of a 400-page education law enacted in December 2015 whose main goal is to increase local control over education standards, school curriculums and standardized testing.
The law was sponsored by Sen. Alexander and received broad bipartisan support. It reverses parts of the No Child Left Behind Act, and requires, for example, that states consider more than just test scores in evaluating schools.
The section of the 2015 act currently in question concerns the longstanding policy that requires school districts to use Title I education funds only to supplement state and local funding and not in place of those funds. Until now, the policy has stopped short of requiring school districts to spend the same money per pupil in Title I schools and in non-Title I schools.
The Education Department has said that 90 percent of the nation's school districts do not have substantial funding disparities between low-income and high-income schools, and would not need to adjust funding.
But Gordon, the Georgetown expert, said the new rule would affect even those school districts that do not need to make immediate funding changes.
"Anything they're thinking about doing, like opening a special arts program in a school, they would need to make sure they're still in compliance," Gordon said. "It really could inhibit innovation in districts because they'd worry about falling out of compliance."