On a sweltering day in August, Claudia Sheinbaum appeared with her mentor, President Andrés Manuel López Obrador, to inaugurate one of the costliest infrastructure projects in Mexico’s history: a $16 billion oil refinery.

The sprawling complex in Mr. López Obrador’s home state, Tabasco, forms the capstone of an energy strategy that he will bequeath to Ms. Sheinbaum, a climate scientist, when she takes the presidency in October.

As countries around the world feverishly turn to clean energy sources, Mexico has placed a colossal bet on fossil fuels, with the costs of that strategy now coming painfully into view.

Mexico’s oil production tumbled to a 45-year low this year, one of the steepest output declines anywhere in the world this century. Blackouts plagued the country after Mr. López Obrador heaped scorn on wind farms that could help satisfy electricity demand. Natural gas imports for the strained grid are soaring, making energy independence an ever more distant dream.

Pemex, the state-controlled oil giant, is now the world’s most indebted oil company after going on a spending spree to build projects. To stave off a default on its nearly $100 billion debt, the company has required multibillion-dollar bailouts using taxpayer money.

The disorder in Mexico’s energy industry lays bare a dilemma that will shape the country’s fortunes — and Ms. Sheinbaum’s presidency — in the years to come. Ms. Sheinbaum, who has a Ph.D. in energy engineering, has signaled that she wants Mexico to pivot to clean energy sources. But the biggest obstacles in her way may be her mentor’s nationalistic energy policies that are fixated on oil — and her reluctance to bump heads with the man who helped put her in office.

“It’s a source of pride to see how Mexican engineers and workers have achieved this feat,” Ms. Sheinbaum said at the refinery’s inauguration.

She barely mentioned her own plans for an energy transition at the event. Instead, Ms. Sheinbaum voiced full-throated support for Mr. López Obrador’s oil-centric policies, calling the refinery, named Olmeca, “majestic,” while blasting previous leaders for exporting Mexico’s oil and opening the energy industry to private investment.

But the refinery, intended to tilt Mexico toward energy self-sufficiency by processing the country’s crude oil into gasoline instead of relying on U.S. refineries, remains far from fully operational, according to the International Energy Agency. Beset by delays and cost overruns, Mr. López Obrador already inaugurated the project once before, in 2022, when it was supposed to start operating in 2023.

Altogether, the Olmeca refinery doubled in cost from its initial $8 billion budget, adding to the financial pressure on Pemex. The company owes financial creditors almost $100 billion, and billions more to service providers that help the company produce oil. Delays in paying these companies led some to halt work this year for Pemex, contributing along with underinvestment in exploration to declining output.

“In one word, it is unsustainable,” Adriana Eraso, a Latin America corporate analyst at Fitch Ratings, said about Pemex’s strain under its debt load.

Neither Ms. Sheinbaum nor Mr. López Obrador responded to requests for comment. Pemex’s leadership also did not respond.

Hints emerged on the campaign trail of Ms. Sheinbaum’s energy plans before she won in a landslide in June. They include building solar plants, pushing Pemex into mining the lithium used in electric vehicle (E.V.) batteries and constructing charging infrastructure for E.V.s.

Ms. Sheinbaum has also proposed a cap on Pemex’s oil production, a change in course that would involve chipping away at one of modern Mexico’s foundational myths, dating to the country’s 1938 nationalization of its oil resources: that Mexico is an oil power, with oil at the core of the economy.

“When I talk to people in my social circle, they tend to believe Mexico continues to be an important oil-producing country,” said Adrián Duhalt, an energy expert at Rice University, citing relatives and friends who work at or have retired from Pemex. “That’s no longer the case when you look at the numbers.”

In the early decades of the 20th century, Mexico was the world’s largest oil exporter. But the country’s crude oil production plunged from 3.2 million barrels a day at the start of this century to about 1.5 million, largely reflecting underinvestment in exploration. While Mexico still exports some crude oil, the country must import everything from natural gas and diesel to jet fuel.

As a result, Mexico’s clout in global energy markets has dwindled as other countries in the Americas — the United States, Guyana and Brazil — rise in prominence. Mexico’s crude oil output is now dwarfed by that of the state of New Mexico, which alone produces two million barrels a day with a population about one-sixtieth the size of Mexico’s.

And yet, schoolchildren still learn about the nationalization of oil in textbooks. Monuments celebrate state control of the oil industry, and polls show broad resistance to any hint of privatizing Pemex. A national holiday on March 18 commemorates the day in 1938 when a leftist president took control of foreign-owned oil assets.

Mr. López Obrador adroitly embraced oil nationalism upon taking office in late 2018, casting attempts by the previous government to open the energy industry to meaningful foreign investment as a sellout.

Prioritizing fossil fuels, he publicly mocked wind turbines after his government canceled auctions for solar projects. His supporters point out political reasons for making such a huge bet on oil.

Octavio Romero, Pemex’s chief executive, contends that Mexico had to pursue costly refinery projects for national security reasons because of the country’s reliance on imports of refined fuels from the United States.

“What happens if for some reason, political or natural disaster-related, the ports for importing gasoline are closed?” Mr. Romero told reporters in April.

Still, the costs of propping up Pemex are climbing. Altogether, Mexican authorities have granted Pemex the staggering amount of at least $70 billion in relief in the form of capital injections and tax breaks since 2019, reflecting how Pemex has gone from providing the bulk of government revenues to requiring repeated bailouts.

Pemex, for its part, remains known for retaining privileges like its own country clubs, hospitals and schools. Some executives enjoy perks like enviable pensions and tuition reimbursement at private universities for their children.

Some argue that the government should withdraw its support for Pemex and let it default, contending that at the moment, the country’s relatively resilient economy could absorb the aftershocks.

Damian Fraser, a former country manager in Mexico for the Swiss banking giant UBS, said that if the authorities did not act now, a default by Pemex down the road could unleash economic chaos by raising borrowing costs for a constellation of companies in a country that has eclipsed China as the largest trading partner of the United States.

“If there is ever a time to let bondholders take a hit on Pemex, this might be it,” said Mr. Fraser, who now runs Miranda Partners, which advises companies on doing business in Mexico. “The government is mainly bailing out oil workers and Wall Street at the cost of expanding Mexico’s social programs.”

But for Ms. Sheinbaum — or any Mexican leader for that matter — withdrawing support for Pemex could also be extremely unpopular. So far, she has made it clear that she has no plans to let Pemex default, seeking instead to refinance Pemex’s debt in hopes of freeing up resources to shift toward clean energy sources.

Ms. Sheinbaum laid out some of her plans on March 18, the 86th anniversary of Mexico’s oil expropriation, framing them as a way to bolster Pemex, keep imported energy at a minimum and avoid increasing energy prices beyond inflation.

She said she would cap Pemex’s oil production at 1.8 million barrels a day, not far from what it is now producing, as a way of “decoupling” energy consumption from economic growth by focusing on clean energy and improvements in energy efficiency.

“The growth in demand must be absorbed by renewable energy sources,” Ms. Sheinbaum said.

Still, specifics remain sparse as to how Ms. Sheinbaum would carry out such a shift, especially at a time when her financial maneuvering room will be limited. Another legacy from Mr. López Obrador will be a budget deficit nearing 6 percent of gross domestic product, the largest shortfall in the past 24 years. Pemex’s debt alone stands at about an additional 6 percent of G.D.P.

The resource nationalism imbuing Mexican politics also raises questions as to how far Ms. Sheinbaum will be able to go in a country where oil remains central to national identity.

“People can’t really rally around lithium like they can around oil,” said Lisa Breglia, a scholar at George Mason University who specializes on Mexico’s oil industry. “Down to Mexico’s last drop of oil, people will still take to the streets.”