President Joe Biden arrives back in the US this week after a foreign tour with a recurring theme: fighting climate change.

But he returns to a Washington where his own party feels increasing anxiety that his administration’s climate agenda will fall short at home.

Bipartisan talks over Biden’s infrastructure proposals — which would spend billions on crumbling roads, bridges and tunnels, as well as record sums on clean energy — are flagging. While Republicans and moderate Democrats try to scale down the package, progressives warn they will withhold support if climate provisions are stripped out.

“If there is no climate, there is no deal,” Jeff Merkley, a Democratic senator from Oregon, said this week. “When the ship sails on infrastructure, energy infrastructure cannot be left on the docks.”

Democratic party leaders are now exploring another path to enact Biden’s climate plan. Chuck Schumer, the Senate majority leader, on Wednesday met his members on the budget committee to find ways to fund greener electricity, zero-carbon vehicles and manufacturing and farming that keeps many climate goals intact.

While potentially more viable, the strategy could also weaken Biden’s climate policies. Legislation would be shoehorned into the Senate’s budget reconciliation process — a special procedure that enables Democrats to use their slim majority but constrains the scope of what can pass.

Column chart of US generation by fuel source, in a scenario where emissions fall 90 per cent by 2040 (TWh) showing Cutting carbon from electric sector would displace coal and gas

Biden has pledged for the US to cut emissions by at least 50 per cent from 2005 levels by 2030. He is aiming for carbon-free electricity by 2035 — a target that would mean none generated by burning coal or natural gas unless their emissions can be captured.

These lofty climate policies were a centrepiece of Biden’s diplomacy on his first international trip. He told G7 leaders in Cornwall that global warming is “the existential problem facing humanity”, and helped launch a $2bn fund for countries to shift away from coal.

In Washington, however, Democrats look unlikely to pass ambitious climate legislation with the support of Republicans given the 50-50 party split in the Senate and rules requiring at least 60 votes to move most important bills.

“I think there is reason to be concerned,” said Dan Lashof, US director of the World Resources Institute, referring to the fate of climate proposals in Congress.

“It was always going to be a challenge, to get investment at the scale that is needed, to turn the corner on climate change,” he added. “Getting very substantial investments in infrastructure and clean energy technologies is crucial to reaching the US emissions targets.”

The reconciliation process proposed by Schumer requires a simple majority vote, but rules limit it to tax and spending measures. Far-reaching initiatives to drive down the US’s 6.5bn tonnes of annual carbon emissions would be in jeopardy.

Using reconciliation would make it hard to establish a “clean electricity standard,” a core part of Biden’s plans to tackle emissions. The standard would set ever-stricter emissions targets for electric utilities, which are the source of a quarter of the country’s greenhouse gas emissions.

“The clean electricity standard is a much harder provision to enact through reconciliation and the reason is pretty simple: it’s a standard,” said Paul Bledsoe, strategic adviser at the Progressive Policy Institute. “Forcing the square peg of a clean electricity standard into the round opening of the reconciliation process will be very difficult.”

One workaround under discussion among Democrats is to pay incentives for clean electricity, achieving some of the goals of the electricity standard while fitting within the guidelines of reconciliation.

“It would involve the federal government becoming a partner in the transition, helping utilities that are making progress at the pace and scale necessary with financial investments,” said Leah Stokes, an assistant professor of political science at the University of California, Santa Barbara.

Other parts of the Biden climate agenda — including expanding tax credits for wind and solar power and energy storage and creating a credit for power transmission lines — would be more straightforward under the reconciliation process. Policies that do not depend on legislation, such as vehicle emissions rules, can be directly imposed by the Biden administration and are expected soon.

Adding urgency to the legislative push are the midterm elections in 2022, when Democrats risk losing control of the Senate or the House of Representatives.

“The rest of the world is intimately familiar with midterm elections and how the US Senate works, because they are concerned that the domestic delivery of the climate promises is imperilled by the toxic politics here,” said Rachel Kyte, dean of the Fletcher School at Tufts University.

Republicans have argued that the infrastructure proposals should focus more on roads, bridges, and construction projects and objected to provisions that would subsidise electric cars and support non-fossil energy.

Meanwhile, not all Democratic senators are aligned. Joe Manchin of West Virginia, a coal-producing state, is likely to have a decisive influence on the shape of any climate proposals as his vote would be needed to pass bipartisan or reconciliation bills.

Energy experts acknowledge that achieving zero-carbon electricity by 2035 would be daunting even if a clean electricity standard was to pass, because of an ageing US grid.

Patrick Luckow, analyst at IHS Markit, expects power demand to rapidly increase over the next decade as more vehicles and home heating systems run on electricity. “When you are adding renewable energy and getting rid of fossil fuels, there is demand growth as well, which makes it more challenging,” he said.

Democrats say Biden needs a win on climate for political reasons as much as environmental ones. “The Democrats can’t risk the failure of Biden’s climate and economic policy. It would cripple the president,” Bledsoe said.