MEXICO CITY — Mexico’s finance minister resigned on Tuesday with a scathing letter that accused President Andrés Manuel López Obrador’s administration of capricious decision-making and conflicts of interest, in a surprise move that could prove a blow to the country’s anemic economy.
The minister, Carlos Urzúa, was widely seen as part of Mr. López Obrador’s inner circle, and served as a crucial reassurance to investors that the leftist president would maintain financial discipline.
In his resignation letter, which he posted on Twitter, he wrote that there were many discrepancies in economic policy, some of them because “in this administration public policies have been made without sufficient evidence.”
Mr. Urzúa, 64, also accused the administration of placing officials with no knowledge of finance in the ministry, motivated by “influential people” in the government “with a patent conflict of interest.”
Mr. López Obrador appointed Arturo Herrera, the deputy finance minister, to lead the ministry. In making the appointment, the president lashed out at his former minister.
“We are committed to changing the economic policy that has been imposed on us for 36 years,” Mr. López Obrador said.
“We cannot continue with the same strategies, you can’t put old wine in new bottles,” the president said.
The suggestion that people close to the president are corrupt could prove especially damaging to Mr. López Obrador, who won a landslide victory a year ago on the promise to sweep away Mexico’s entrenched graft.
During the presidential campaign last year, Mr. Urzúa, who has a doctorate in economics from the University of Wisconsin, spent months meeting with investors who were wary of Mr. López Obrador’s populist rhetoric to persuade them that the new government would keep tight control over finances.
Since taking office last December, Mr. López Obrador has been adamant that Mexico would not increase debt. Instead, he has said that his administration would be able to pay for ambitious social programs with the savings earned from rooting out corruption.
In practice, though, the finance ministry has made sharp cuts in spending on other important programs, like higher education and environmental protection.
Several of Mr. López Obrador’s moves raised concerns among investors. He canceled a new Mexico City airport a third of the way through its construction, and then spent billions of dollars repaying bondholders. And he has halted the opening of Mexico’s energy industry to private investment in favor of government spending on Mexico’s debt-ridden oil and electricity companies.
Mr. López Obrador also has expensive plans to build a new oil refinery and a train through the Yucatán Peninsula.
Those decisions have led ratings agencies to revise downward their outlooks for Mexico’s creditworthiness. The most recent survey of private sector economists carried out by Mexico’s Central Bank showed that, on average, they expected growth to reach just over 1 percent this year.
The problems that led Mr. Urzúa to quit will place enormous pressure on Mr. Herrera, his successor, said Christopher Wilson, the deputy director of the Mexico Institute at the Wilson Center.
“His situation is virtually untenable,” Mr. Wilson said. “He is being asked to lead a finance ministry that includes officials that shouldn’t be there, and he is being asked to implement policies that aren’t well-founded.”
Mr. López Obrador has made his suspicion of technocrats a hallmark of his presidency, pushing out experts at regulatory agencies and throwing rhetorical barbs at the United States-trained economists who have set economic policy over the past three decades.
The finance ministry had seemed somewhat insulated from the tension between the president’s political instincts and the need for technical expertise, until Mr. Urzúa’s letter pushed the conflict there into the open.
Mr. Herrera has disagreed publicly with Mr. López Obrador in the past, telling the Financial Times in a March interview that construction of the oil refinery would be delayed in order to direct money to the state oil company Pemex. The president then contradicted Mr. Herrera and said the project would go ahead as planned.
The question going forward is whether Mr. Urzúa’s public criticism would have any effect on the president, said Mr. Wilson.
“Did the president himself learn a lesson from this?” Mr. Wilson said. “If managed properly, this is, in some way, an opportunity for there to be a recalibration in terms of policymaking.”