Senate negotiators say they are close to finalizing a $1 trillion infrastructure deal, but funding for transit has emerged as one of the last big obstacles. Republicans and Democrats are sparring over a long-standing ratio of funding for highways and transit known as the 80-20 split.

The split — giving transit one dollar for each four that highways get — has its roots in the 1980s but has only been sustained by an uneasy truce between lawmakers. Republicans have sometimes proposed scaling back transit funding, while Democrats have hoped to increase its share. The dispute has arisen again as a group of senators tries to wrap up the infrastructure package.

In recent days, uncertainty over funding for buses and rail lines prompted some Democrats — who see spending on transit as a way to achieve environmental and racial-equity goals — to indicate that their support for the overall package was wavering. Some Republicans, meanwhile, are quick to highlight that transit agencies received $70 billion in coronavirus relief money, arguing they do not deserve another infusion of cash.

Why is federal transportation spending split the way it is?

The 80-20 approach has its roots in a 1982 effort to raise the federal gas tax by 5 cents. To gain the support of transit advocates, Congress agreed to deposit 1 cent in a new transit account, with the other 4 cents going to highways. In the years since, the split has become a shorthand for the government’s broader approach to funding transportation.

If Congress simply continued to fund transportation at current levels and adjusted for inflation each year, spending would hew close to the ratio: Highways would get about 79 cents of every dollar, with transit getting the remaining 21 cents in five years.

The original bipartisan framework from late June called for an additional $49 billion in transit funding and $109 billion for roads. Added to current funding, that would seem to shift the split to about 75-25, although publicly released spending breakdowns do not include the full details.

But what ultimately gets counted toward the ratio is open to debate, and aides familiar with the infrastructure negotiations say there has been a dispute about whether spending plans would maintain the long-standing split, or depart from it.

At points in recent months, the differences in the parties’ proposals have been especially stark. President Biden’s original American Jobs Plan called for $85 billion in transit funding, and probably would have pushed the split to 68-32. An early Republican offer proposed paring back transit funding slightly.

White House press secretary Jen Psaki said Friday that the White House would not meddle in the dispute, adding that “transit funding is obviously extremely important to the president.”

Why does the dispute matter?

The question of transit funding underscores different ideas among Republicans and Democrats about what infrastructure spending should achieve. Democrats argue that boosting transit funding would encourage more Americans to use buses and trains, reducing carbon emissions from cars and tackling congestion without building new road lanes.

“We view the 20 percent as the floor,” said Paul P. Skoutelas, president of the American Public Transportation Association. “The absolute minimum.”

But Republicans do not see transit as feasible in rural areas and question any new funding after the coronavirus relief infusion.

“It’s without precedent in terms of the generosity for transit and departs from all of the pre-covid traditions,” Sen. Patrick J. Toomey (R-Pa.) told the Hill this week.

Toomey is the top Republican on the Senate’s Banking, Housing and Urban Affairs Committee, which is responsible for transit policy. Other Senate committees that work on transportation have struck bipartisan deals on long-term funding bills for roads, rail and safety programs in recent weeks, helping the broader infrastructure negotiations. But Toomey and the banking committee’s chairman, Sen. Sherrod Brown (D-Ohio), have not reached an agreement, which has only served to complicate things.

On Thursday evening, Brown issued a joint statement with Sen. Thomas R. Carper (D-Del.), who chairs the committee responsible for roads, outlining their concerns.

“Highway and transit programs are always reauthorized together, and we will not break precedent this year,” they wrote. “Robust funding for transit must be included in the legislation. We will not support any package that neglects this fundamental part of our nation’s infrastructure.”

What approach has the House proposed for transit spending?

The House this month passed a transportation and water funding bill, known as the Invest in America Act, that forms the core of House Democrats’ pitch on infrastructure. It proposes $109 billion for transit and about $340 billion for roads, a split of roughly 75-25. The bill also would make tweaks to how transit funding is spent, including an effort to reward more frequent service and changes to the rules for grant programs.

Rep. Peter A. DeFazio (D-Ore.), the lead architect of the bill, has been warning this week that House Democrats will want to see a host of environmental measures from their bill included in any final deal.

In a memo comparing the House bill to the Senate’s pitch, he highlighted the uncertainty: “Transit: $109 billion (INVEST) vs. ?”

“The Senate has yet to produce a transit title,” DeFazio wrote. “But Republicans in the Senate have publicly called for underfunding transit so that it doesn’t even get the traditional 20 percent.”