As the war in Ukraine has raised oil prices and launched Western leaders on a global hunt for new sources of energy, environmentalists have tried to leverage the chaos in energy markets to move the United States off fossil fuels.

But there’s a problem: Despite major advancements in wind and solar power — and renewed investor enthusiasm — Democrats, who narrowly control Congress, have yet to devise a legislative strategy that could accelerate this shift. And without major climate legislation soon, President Biden is likely to lose his opportunity to transform the nation’s energy mix during his first term.

Experts say that for the United States to supercharge renewable power, lawmakers have to lower the upfront costs of building wind turbines, solar arrays and energy battery storage through federal tax breaks and other incentives. But congressional Democrats are struggling to revive the climate provisions of their roughly $2 trillion package formerly known as the Build Back Better Act, as their focus has shifted in recent days to bills that narrowly aim to respond to the rising cost of gasoline and diesel fuel.

“I worry all the time we’re going to miss this moment to take bold action on climate,” Sen. Tina Smith (D-Minn.) said in an interview Thursday. “Taking steps right now to lower energy prices for Americans … I think is a good thing. But that’s not going to be a real solution if we don’t come up with long-term solutions.”

Sustained opposition by Sen. Joe Manchin III (D-W.Va.) has bogged down a bill that includes about $300 billion of tax credits for wind, solar and nuclear energy producers and other incentives to boost the clean hydrogen and battery storage industries.

Last month, Democratic Sens. Ron Wyden (Ore.), Patty Murray (Wash.) and Thomas R. Carper (Del.) — who oversee key tax, education, and health and environment committees — each tried to engage Manchin in formal, detailed negotiations, according to two people familiar with the matter who spoke on the condition of anonymity to describe the private outreach. But Manchin wouldn’t engage, they said: His focus remained fixed on other fiscal matters and on curbing inflation.

Without a strategy to pass climate legislation, Democrats have embarked on a new messaging campaign to rebrand their economic agenda as an effort to save Americans’ money, lower the deficit and fight inflation. The legislative push, which coincides with a separate, nascent effort to penalize oil giants that gouge customers with high prices, could mark a new wave of scrutiny targeting the energy sector.

Manchin remains a holdout on a broader bill. He has indicated that he could support a scaled-back measure that includes several clean energy provisions, as well as ones to lower prescription drug prices, make changes to the tax code and reduce the deficit. But at an energy conference last week, he attacked a key piece of the administration’s climate strategy, saying he was “very reluctant” to see the growth in electric cars.

Smith said she still thinks the clean energy tax credits and other climate provisions in the act remain broadly popular among Democrats. But she said there have only been “casual conversations” between lawmakers and Manchin on the sticking points. “There isn’t any organized negotiation,” she said.

Clean sources of electricity have made significant strides even during the coronavirus pandemic, when energy demand shrank and U.S. oil and gas companies cut production. According to the American Clean Power Association, a trade group for renewable energy companies, there is now enough clean energy capacity in the United States to power 56 million homes.

But while 2021 was the second-biggest growth year for combined wind, solar and energy storage installations in the United States, the nearly 28 gigawatts of new clean energy amounted to only 45 percent of what the trade group said is needed to stay on track to reach Biden’s goal of achieving a carbon-free power sector by 2035. Another 11 gigawatts’ worth of clean energy projects that were supposed to come online last year ran into regulatory, financial and supply delays.

A report published last month by the U.S. Energy Information Association offered a sobering look at the country’s energy transition. Without changes in federal policy, oil and gas will still be the leading sources of U.S. energy in 2050 — the same year by which scientists have said humans must reach net zero emissions and stop fueling climate change

“There’s just a massive gap now from where we need to be and the trajectory we’re heading down,” said Jason Bordoff, the founder and director of Columbia University’s global energy institute. Bordoff cited political divisions in Washington and across the nation that he said have made it impossible for all 50 members of the Senate Democratic caucus and even a single Senate Republican to support major climate legislation.