MIAMI (AP) — Democrats in Congress are sounding the alarm over what they claim is mounting evidence that Mexico’s chief prosecutor — a vital partner of U.S. law enforcement — is assailing the nation’s independent judiciary and selectively targeting for prosecution opponents of President Andrés Manuel López Obrador.

In a harshly worded letter sent Wednesday to Secretary of State Antony Blinken and Attorney General Merrick Garland, Sen. Bob Menendez, the powerful chairman of the Senate Foreign Relations Committee, and three colleagues call on the Biden administration to raise their concerns directly with their Mexican counterparts.

“López Obrador’s tenure has been marked by an increasing pattern of seemingly selective prosecutions disproportionately targeting government critics,” according to the letter, a copy of which was obtained by The Associated Press. “President López Obrador’s efforts to advance legitimate accountability initiatives must strengthen, not dismantle, democratic institutions and the rule of law.”

The letter, which is bound to stir the ire of López Obrador and his allies, focuses on a number of questionable actions and what they consider “personal vendettas” pursued by Attorney General Alejandro Gertz Manero.

The top prosecutor, a close ally of the leftist president, first caught the attention of American officials after the Trump administration dropped criminal narcotics changes against former Defense Minister Salvador Cienfuegos and returned him to Mexico on promises he would be investigated at home.

But the probe against Cienfuegos was quickly closed and Gertz Manero later threatened to press charges of his own against U.S. prosecutors accustomed to working hand-in-glove with Mexican law enforcement to dismantle the country’s powerful cartels.

“We urge you to give serious consideration to the risk of a weakened, politicized justice system in Mexico,” according to the letter, whose signatories include Vermont Sen. Patrick Leahy, chairman of the appropriations committee, and Senators Benjamin Cardin of Maryland and Jeffrey Merkley of Oregon.

López Obrador was elected in 2018 on a promise to sweep clean Mexico’s notoriously corrupt politics and likes to tout his commitment to austerity by taking commercial flights and living in a modest apartment instead of the luxurious presidential residence.

But critics say such moves are populist stunts and distract from a worrisome accumulation of power, failure to deliver on campaign promises and repeated attacks on opponents that undermines the rule of law in the U.S.’ second largest trade partner. This Sunday, his supporters are expected to give him another boost when they head to the polls for a first-of-its-kind referendum on whether he should be allowed to finish his six-year term — something that has never been in doubt and which many see as a sideshow.

The Democrats in their letter fault López Obrador for publicly attacking a judge who ruled against his energy policies, pushing a seemingly unconstitutional plan to extend the term of a sympathetic Supreme Court chief justice and calling for the resignation of Mexico’s top electoral court.

They also raise concerns about criminal charges pressed against Ricardo Anaya, a prominent conservative opponent of López Obrador.

Anaya, who finished second in the 2018 presidential election, was accused last year with money laundering in connection to an alleged bribe he took in exchange for his support on an energy reform bill. The charges are based on the testimony of the former head of Mexico’s state run oil company, who claims that on the instructions of López Obrador’s predecessor he paid lawmakers, including $525,000 to Anaya, to vote for the overhaul.

Some have questioned the strength of the evidence, given that the alleged bribe was paid months after the reform was approved, when Anaya had already left office. Anaya has since fled Mexico and is living in the U.S.

López Obrador has dismissed claims of score settling in the Anaya and other cases as “lies” and “falsehoods” promoted by opponents to weaken his rule.

There was no immediate response from either his or the attorney general’s office when the AP sent them a copy of the letter.

Under Gertz Manero’s leadership, the Mexican prosecutor’s office has also failed to seriously investigate allies of the president, according to the letter. These include allegations of money laundering and finance law violations against the president’s brother, who was caught on video receiving cash from a campaign supporter. López Obrador has defended the contributions as legitimate.

The Democrats also accuse Gertz Manero of pressing personal matters while in office.

These include trying to lock up 31 scientists in a maximum security prison because he claims they improperly received about $2.5 million in government funding years ago. The laws at the time allowed such funding, and the researchers say it wasn’t misspent. Critics say the charges are payback for the researchers’ refusal to recognize Gertz Manero’s own academic credentials.

The attorney general also can be heard in a recently leaked recording of a conversation with a colleague cursing a Supreme Court justice. In the recording, Gertz Manero claims the judge won’t heed to demands that the top court keep in jail a niece he blames for the death of his older brother, who died while in her care.

Gertz Manero has acknowledged obtaining an advance copy of a proposed Supreme Court opinion recommending the relative be released — something that struck many as a potential conflict of interest.

But he claims he received the court ruling because he was taking action in the case as a family member of the deceased, not as attorney general.

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AP Writer Mark Stevenson in Mexico City contributed to this report.