MEXICO CITY — President Andrés Manuel López Obrador lashed out Friday at the U.S. ambassador after he warned that Mexico’s democracy faced a “major risk” from a plan to dismantle the federal judicial system and allow voters to pick judges.

The war of words marked the sharpest public dispute between the countries since President Joe Biden took office. López Obrador, a populist and longtime icon of the left, plans to push through Mexico’s Congress a constitutional change next month to institute direct election of judges — including those on the Supreme Court.

U.S. Ambassador Ken Salazar said at a news conference Friday that the change would harm Mexico’s democracy and “threaten the historic trade relationship” between the United States and its No. 1 commercial partner. The new system would also make it easier for drug cartels to influence the judiciary, Salazar said.

The dispute comes at a delicate moment for the Biden administration and the presidential campaign of Vice President Kamala Harris. Mexico has played a crucial role in keeping migrants from reaching the United States, handing a major victory to the Democrats as they try to stop former president Donald Trump from riding the border issue to victory in November. Apprehensions at the U.S. border have plunged by half since January, as Mexico has ratcheted up enforcement.

Critics have charged that the Biden administration has been so eager to win cooperation on migration over the past few years that it has tiptoed around sensitive issues like the weakening of Mexico’s democratic institutions. U.S. officials have denied that, saying they speak frankly about such concerns — in private.

This week, the squabble burst into the open.

López Obrador said at his daily news conference Friday that the U.S. ambassador’s statement reflected “a crude interventionist attitude.” He underscored a history of American political meddling in Latin America, including two invasions of Mexico.

“We don’t accept interventionism. We will not allow any representatives of foreign governments to interfere in issues that are ours to resolve,” the president declared. He said the Foreign Ministry would send a protest note to the U.S. Embassy.

López Obrador has said the plan is aimed at rooting out corruption in the judiciary. Until now, he hasn’t had enough votes in Congress to change the constitution and redesign the judicial system.

But the president’s Morena party won a massive victory in June elections, including ample majorities in Congress. Those lawmakers take office on Sept. 1, a month before the inauguration of the new president, Claudia Sheinbaum, a protégé of López Obrador’s. He plans to use his last month to push through the change.

The president has accused federal courts of blocking many of his most far-reaching proposals, such as increasing the state’s role in the electricity industry and giving the military control over the national guard, which was created as a civilian institution.

According to the legislation, hundreds or perhaps thousands of judges would be elected by voters starting next year. Candidates would be proposed by the president, the legislature and the judicial system, which would screen them for suitability.

Salazar said the plan “removes the necessary requirements of having the highest qualifications for judges” and reduces the required years of experience. He warned that the changes would “create turbulence as the debate over direct election will continue over the next several years.”

The proposed judicial overhaul has sparked widespread concern. More than 50,000 workers in the judicial system have walked off the job in an indefinite strike. Business leaders are warning about a slump in foreign investment. Salazar said the new system could damage the bilateral trade relationship, “which relies on investors’ confidence in Mexico’s legal framework.” Canada’s ambassador, Graeme Clark, also said Thursday that investors were concerned.

“They want a judicial system that works, if there are problems,” he said.

The Wall Street firm Morgan Stanley downgraded its investment recommendation for Mexico this week, saying that the proposal “should increase risk.” Citibanamex said that the ruling party could alter the constitution and create a “new regime,” effectively “canceling liberal democracy, based on the rule of law” and checks and balances. The peso has lost ground against the dollar since the June elections.

Luis Rubio, president of the think tank México Evalúa, said he doubted the pressure would force the president to change course. “He’s certain that what matters is this project of total control into the future,” he said.

López Obrador and Sheinbaum have noted that they won a broad mandate in the June elections and that Mexicans have complained for years about corruption in the judicial system.

“Investors shouldn’t be worried. On the contrary, we will have a better justice system in Mexico,” Sheinbaum said this week.