The Senate infrastructure bill approved last week includes an unprecedented boost in funds for a Transportation Department program to reduce harmful emissions from buses.

But the provision adding billions to the Low or No Emission Vehicle Program has an unusual requirement: At least $1.4 billion must be spent on buses that pollute.

In a legislative turn of phrase with far-reaching implications, the bill says that for at least a quarter of spending in the $5.6 billion program, the secretary of transportation shall “only consider eligible projects related to the acquisition of low or no emission buses or bus facilities other than zero emission vehicles and related facilities.”

That would represent a sharp break from recent practice. Over the past two years under Presidents Biden and Donald Trump, at least 95 percent of funds in the program went toward electric buses, which have zero tailpipe emissions. Canada launched a $2.2 billion Zero Emission Transit Fund last week with no money for polluting buses. And California has mandated that transit agencies purchase only zero-emission buses starting in 2029.

The provision appears about halfway into a 2,702-page infrastructure bill released to the public nine days before it was approved Aug. 10, leaving little time for those outside closed-door negotiations to learn of its existence. But a closer look at the origin of the 42-word passage shows the difficulty of getting the federal government fully behind efforts to address emissions and climate change in a politically divided nation deeply reliant on fossil fuels.

The inclusion of the provision requiring polluting buses followed lobbying to shape the program by the natural gas industry — which wants to fuel dramatically more U.S. vehicles in the coming decades — and the insistence of an enthusiastic industry advocate, Sen. Patrick J. Toomey (R-Pa.), according to federal disclosures and interviews.

Natural Gas Vehicles for America, a group representing major utilities and others in the industry, said in a statement that its lobbying on the Low or No Emission program was meant to help transit agencies quickly purchase the “cleanest available technologies that suit their needs. Natural gas buses are more affordable, more reliable and deliver greater environmental benefit than electric buses,” the advocacy group said.

Members of Toomey’s staff said he was responsible for the 25 percent mandate for emitting buses.

“It is called the ‘low-no’ emission vehicle program, not the ‘no-emission’ vehicle program,” said Brad Grantz, staff director for Republicans on the Senate Banking Committee, where Toomey is the highest-ranking Republican. “The way the program has been structured has been to preclude the purchase of low-emission vehicles powered by natural gas or by other alternative fuels.”

Among the limited circle of Democrats who have learned of Toomey’s provision, reactions ranged from befuddlement to resignation. A Capitol Hill aide who was not authorized to discuss the matter publicly said subsidizing the fossil fuel industry through a program meant to wean buses from the industry symbolizes the GOP’s approach to the climate.

But the transportation chief in a Democratic state, who requested anonymity to speak freely, argued that difficult concessions were inevitable to get a deal. “More money to transit is more money to transit,” the official said.

The measure benefiting buses running on fossil fuels comes as the Biden administration is seeking to cut greenhouse gas emissions in half by 2030. It also comes amid criticism from the left that the bipartisan Senate infrastructure bill embraced by the White House doesn’t do enough to shift the federal funding balance away from highway spending and toward transit to reduce emissions.

On stops around the country, including in Texas last week, Transportation Secretary Pete Buttigieg has emphasized successes in the bill and its potential for transformational impacts, including what he called the biggest transit investment in the nation’s history.

Asked about the bus provision, his office said in a statement that “the modification to the Lo-No program did not originate from the administration, but we appreciate that bipartisan legislation often reflects compromise.”

Noting that Biden set a goal during the presidential campaign that new American-built buses would have zero emissions by 2030, the statement said the Transportation Department would use the tools it has to support the shift toward zero-emission vehicles. The Low or No Emission program would have $4.2 billion to support that shift over five years, the department said. That compares to $536 million in spending over the past five years.

Senate committees came to a bipartisan agreement over many policy issues that eventually became part of the $1.2 trillion infrastructure bill. A surface transportation funding bill passed unanimously by the Environment and Public Works Committee in May became a pillar of the bipartisan compromise, which Biden has held up as an example that the U.S. democratic system can work.

But Toomey, who has railed against transit spending, did not find similar common ground with Sen. Sherrod Brown (D-Ohio), who chairs the Senate Banking, Housing and Urban Affairs Committee that oversees transit. During a hearing this spring, Toomey referred to “transit bailouts” and asked: “Does mass transit continue to make sense in every U.S. city at its current scale?”

In the weeks after the framework for a bipartisan infrastructure deal was announced in June, billions were cut from transit. Still, the five-year total of nearly $107 billion for transit was about 65 percent above what was expected if spending remained flat in that period, according to Jeff Davis, a senior fellow at the Eno Center for Transportation.

The mandate for emitting buses emerged during negotiations, Senate aides said. A Democratic Senate staff member familiar with the negotiations said, “Sen. Toomey insisted on the provision that benefits natural gas buses.” Grantz said Toomey’s bus provision was “just one of a myriad of items” in the negotiations.

In a statement, Brown’s office said the senator was proud that lawmakers secured “by far the largest-ever federal investment in zero-emission buses” that will replace thousands of outdated diesel buses, create manufacturing jobs and improve air quality.

Outside of the Low or No Emissions program, transit agencies also can purchase buses using funds from other Federal Transit Administration programs. The agency doesn’t limit purchases to a particular type of engine or fuel.

The percentage of the nation’s roughly 70,000 buses that run on diesel “has declined as more environmentally friendly natural gas and hybrid buses have been introduced,” according to an American Public Transportation Association survey. In 2019, diesel powered more than 40 percent, natural gas 29 percent and hybrids 18 percent, the survey found.

Replacing older diesel buses with those that run on natural gas can reduce key pollutants, including nitrogen oxides and particulate matter. Diesel soot is a carcinogen. But data collected by the Argonne National Laboratory shows that natural gas buses have nearly the same level of greenhouse gas emissions as diesel buses in many circumstances.

The natural gas used in some buses — including many in California — is derived from renewable sources, such as by capturing methane from waste on dairy farms, which has emissions benefits.

Battery electric buses and fuel-cell electric buses, which use hydrogen and oxygen to make onboard electricity, have zero emissions. But producing the electricity to charge an electric bus can create greenhouse gas emissions, with the amount varying dramatically based on whether electricity is produced by solar, coal or some other mix.

The 2015 law establishing the details of the Low or No Emission program leaves room for discretion when the transportation secretary is selecting which projects to fund. It defines a “low or no emission vehicle” as either a zero-emission vehicle or one the secretary determines “sufficiently reduces energy consumption or harmful emissions, including direct carbon emissions, when compared to a comparable standard vehicle.”

There are challenges to replacing older, dirtier buses with those that don’t have emissions, including the higher cost of electric buses, their limited range, the infrastructure and logistics needed for charging, and the ways operators drive.

But transportation officials say those problems demonstrate the types of challenges the Low or No Emissions program was intended to help communities work through. There were about 1,000 electric buses in use nationwide by the end of last year, with another 1,700 zero-emission buses on order, according to an inventory by transportation group Calstart. There are more than 13,000 natural gas buses on U.S. roads, according to a Natural Gas Vehicles for America analysis of federal data that includes smaller buses.

Toomey has argued that consumers and manufacturers are benefiting from an “energy renaissance” in Pennsylvania, which he said produces roughly 20 percent of the nation’s natural gas and has seen sharp drops in residential and wholesale energy prices. He said the state has also seen falling carbon dioxide emissions, as power plants are fed by fracking, the practice of injecting water, sand and chemicals into bedrock to collect gas.

The shift from coal to natural gas reduced U.S. emissions, and the natural gas industry has tried to convince Congress the same would be true for the transportation sector. One industry group told congressional leaders that half of all cars, buses and heavy trucks could be running on natural gas by 2050 — a notion rejected by many transportation and emissions experts.

The move in the Senate to mandate spending on emitting buses comes as California, which has become a major natural gas bus market, is trying to move away from such vehicles. The state’s powerful California Air Resources Board in December 2018 mandated that transit systems stop using natural gas and other buses with tailpipe emissions by 2040, a decade after a ban on purchasing them takes effect.

As part of its rollout plan, the Los Angeles County Metropolitan Transportation Authority says it will purchase battery-electric buses and convert about 757 natural gas buses to battery-electric, with a goal of an all-electric fleet by 2030, although it says questions about funding remain unanswered.

Proterra, a California-based electric bus manufacturer, said its buses are used by 130 transit agencies, from Juneau, Alaska, to Austin, and have a low cost of ownership. With no tailpipe emissions, one bus saves 229,000 pounds of greenhouse gases each year compared with a diesel bus, 219,000 pounds compared with a compressed natural gas bus and 163,000 pounds compared with a hybrid bus, the company said.

Grantz said debate over the 25 percent set aside for low-emission buses was not particularly contentious.

“This was not one of those intense, vitriolic points of the negotiation,” Grantz said. He said multiple senators, Republicans and Democrats, shared Toomey’s view “that EV buses are not a great fit” for transit agencies in extreme climates, pointing to “one Democrat from Vegas on the Banking Committee.”

A spokeswoman for Sen. Catherine Cortez Masto (D-Nev.) said the senator “was not involved in any talks about this provision and did not support the 25% carve-out in any negotiations.”

After securing the $1.4 billion mandate for buses with emissions, Toomey voted no on the infrastructure bill. He said spending $107 billion on transit was excessive, and called the bill “too expensive, too expansive, too unpaid for.”