correction

An earlier version of this article misrepresented the position of the United Auto Workers. The union has not signed on to the Biden administration plan and is still in discussions. The article has been corrected.

When it comes to meeting President Biden’s climate goals, the math is clear: Half of all cars and SUVs sold in 2030 need to be electric.

Next week, the president and major carmakers plan to promise to reach at least 40 percent by then — potentially rising to the 50 percent mark with generous federal investment.

The voluntary pledge, which is still under negotiation, highlights the challenge the White House faces as it seeks to translate the president’s bold rhetoric into reality. With United Nations climate negotiations just 3½ months away, the administration is struggling with how to transform the transportation sector, America’s biggest driver of carbon emissions.

The talks underscore the difficulty of reversing course after four years of environmental rollbacks. While Biden’s team can write new rules, it won’t fully offset the climate-warming pollution caused by the Trump administration’s decision to weaken standards on cars.

The administration also faces the tough task of trying to secure an agreement with the nation’s largest auto union, the United Auto Workers, to significantly boost sales of electric vehicles. “The UAW is still in discussions and has not reached agreement at this point,” union spokesman Brian Rothenberg said.

Jody Freeman, who directs Harvard Law School’s environmental and energy law program, said in a phone interview that she was confident the White House could develop a framework by the end of the year to reach its goal of cutting U.S. emissions in half by 2030 compared to 2005 levels. But each of these policies faces political and legal obstacles, she added.

“There’s a battle on every front. There’s a battle in Congress. There’s a battle in the courts. There’s a battle against time,” she said. “And that’s what it means to run the country.” The administration’s plan to transition the nation’s auto fleet to electric vehicles running on renewable power is critical to making the United States carbon neutral by 2050. The nation needs to stop selling gas- and diesel-powered cars by 2035, which means half of new vehicles in 2030 must be electric.

“You’ve got to be in the 50 percent-plus range by 2030. And it’s got to be firm,” said Chet France, a former senior executive in the EPA’s Office of Transportation and Air Quality who now works as a consultant for the Environmental Defense Fund. “If we’re not serious about achieving that, then we’re not serious about our long-term climate goals.”

Automakers, however, have emphasized that they need a major federal investment in a nationwide network of charging stations for electric vehicles, along with billions more in tax credits and grants to help retool their factories. One of the biggest barriers to selling battery-powered cars is overcoming drivers’ fear of not being able to find spots to plug in and recharge, and right now there are just 100,000 public charging outlets nationwide, with only about 18,000 boasting fast chargers capable of rapid fill-ups. The distance that current models can go on one charge is about 130 miles to over 400 miles, with the typical range 250 to 300 miles.

It’s unclear if Congress will spend enough on charging stations to assuage automakers. On Wednesday, Senate Democrats and Republicans clinched a deal providing $7.5 billion for building vehicle chargers along highways and other corridors — only half of what Biden initially called for in the spring. Similarly, funding for zero-emissions buses was cut to $2.5 billion.

Even with the promise of new funding, carmakers are hesitant to make a binding commitment on zero-emission vehicles by the end of the decade.

Next week the nation’s three biggest automakers — Ford, General Motors and Stellantis, formerly Fiat Chrysler — are expected to endorse the goal of having electric vehicles make up between 40 percent and 50 percent of their new car sales by the end of the decade, according to three individuals briefed on the plan who spoke on the condition of anonymity because the deal has not been announced. The exact range is still being discussed, these individuals said, and the upper end would be contingent on significant government support.

White House officials and Democratic lawmakers are also working on legislation to ensure that the $1 billion in penalties Stellantis owes the U.S. government for failing to meet current federal mileage standards will help fund the transition to electric vehicles.

In addition, the Environmental Protection Agency and Transportation Department will propose tightening tailpipe emission standards for cars and light trucks between model years 2023 and 2026.

Rep. Debbie Dingell (D-Mich.) who has been working with industry, union and administration officials on mileage standards, said in a phone interview Wednesday that there’s a way to forge a compromise.

“It’s not the environment versus jobs,” Dingell said. “You can do both, and people are working toward that, while keeping production in the U.S. and keeping the U.S. at the forefront of the global auto market.”

Both California and Europe, meanwhile, are moving more swiftly to shift their auto sectors off fossil fuels. California Gov. Gavin Newsom (D) signed an executive order in January mandating the phaseout of the internal combustion engine in passenger vehicles by 2035. Europe just adopted a plan this month requiring that zero-emission vehicles make up 55 percent of its passenger auto fleet by 2030.

Now analysts say the United States could risk falling even further behind China and Europe in the race to produce batteries and other key equipment if domestic manufacturers don’t move swiftly enough to shift to electric vehicles. At the moment, only 15 percent of approximately $340 billion in electric vehicle investments is destined for the United States, according to the International Council on Clean Transportation.

American consumers, for their part, continue to favor large, gas-guzzling vehicles, even as hybrid and electric vehicles make modest gains in the market.

Sales of zero-emission vehicles accounted for 3.8 percent of total vehicle sales in June, up from 1.5 percent a year earlier. Hybrids also made gains, making up nearly 5 percent of overall sales.

But the top-selling U.S. vehicles continue to be light trucks. So far this year, all but three of the top 10 are either pickups or SUVs — and the three car models rank toward the bottom of that list.

The U.S. auto fleet has failed to make the efficiency gains President Barack Obama sketched out just before leaving office, when he finalized rules calling for a 5 percent improvement year-over-year through model year 2025. Trump officials reversed that policy and enacted a rule that requires a 1.5 percent annual efficiency improvement.

The rollback split the auto industry, with five automakers — Ford, Honda, Volkswagen, Volvo and BMW — ultimately deciding to meet California’s call for tightening their fleet’s average mileage by 3.7 percent a year. The Biden team will model some of its near-term tailpipe targets on the California deal.

Yet it is hard to make up for lost time. Even if the California deal was adopted nationwide through 2026, the reduction in carbon dioxide equivalent emissions would fall short by about 300 million metric tons of what would have been achieved if the country had stuck with the Obama-era standards, according to an analysis by the Union of Concerned Scientists. That’s equal to annual carbon dioxide emissions of 75 coal-fired power plants.

“The industry should have been well prepared,” said Dave Cooke, a senior vehicles analyst with the group. “We’ve had this fight so many times.”

Biden officials have made some progress, however, in forging a near-term compromise on tailpipe emission standards with automakers. GM, which sided with the Trump administration in litigation over a rollback of Obama-era standards, signaled that it could support tougher pollution limits.

GM told EPA Administrator Michael Regan in June that it can support limits that other car manufacturers negotiated with California — as long as it can meet those targets through the sale of electric cars and not just through improvements to its gas-powered line of vehicles.

“We’re skipping hybrids,” GM chief executive Mary Barra said in a video call Thursday, “and going straight to all-electric vehicles because that’s the end solution.”

She added: “We can meet the spirit of that framework, and that’s the discussion we’re having that I think has been very, very positive. So it wasn’t a big left turn.” Earlier this year, GM said it “aspires” to eliminate tailpipe emissions from its new light-duty fleet by 2035.

Looking even further out, Ford announced in May that it expects 40 percent of its sales to be of electric vehicles by 2030.

But the Biden administration risks arriving in Scotland for an international climate conference in November without having enacted several policies key to meeting its new climate goals. That list includes curbing emissions from the power sector, which would require a new regulation or law.

That puts ever more pressure on Biden and his allies in Congress to include far-reaching climate measures in a $3.5 trillion budget reconciliation bill that could be passed with Democratic support alone. That package includes ​​a slew of clean energy tax breaks and requirements on electricity providers to shift to wind, solar and other low-emissions forms of energy.

But with a razor-thin majority, Senate Democrats can’t afford to lose a single vote on the budget bill. Passage is far from guaranteed. Moderate Sen. Kyrsten Sinema (D-Ariz.), for instance, balked at the price tag. “I do not support a bill that costs $3.5 trillion — and in the coming months, I will work in good faith to develop this legislation,” she said in a statement Wednesday.