Mexico's government is trying to block the execution of Edgar Tamayo Arias, saying it would violate international law.

HOUSTON — Secretary of State John F. Kerry and a former Texas governor are part of an international coalition trying to halt Texas' execution of a Mexican citizen this week.

Edgar Tamayo Arias, 46, is to be put to death Wednesday for fatally shooting Houston Police Officer Guy Gaddis in 1994.

Gaddis, 24, had been flagged down near a nightclub by a man who accused Tamayo of robbing him. The officer arrested Tamayo, handcuffed him and put him in the back seat of his patrol car. He was driving away when Tamayo drew a concealed pistol and shot Gaddis three times in the back of the head.

On Tuesday, a federal judge in Austin rejected Tamayo's request for an order that would have prevented Gov. Rick Perry and the parole board from considering his clemency petition until the fairness of the state's clemency process could be reviewed. The judge found that the clemency process satisfied constitutional requirements and did not violate Tamayo's right to due process of law.

Tamayo's attorneys vowed to keep fighting.

"The Texas clemency process is the weakest in the nation, in the state that executes the most. Allowing Mr. Tamayo's fate to be decided by a board that has refused to provide meaningful consideration of evidence that Mr. Tamayo has mental retardation and that his trial was fundamentally unfair as a result of the violation of his consular rights is an affront to what clemency is supposed to be," the attorneys said in a statement.

They have petitioned Perry to grant a 30-day reprieve and the parole board to commute Tamayo's death sentence to life in prison.

Tamayo, a laborer from Morelos state, Mexico, was in the U.S. illegally at the time of his arrest. Advocates say he was not informed of his right to diplomatic assistance under an international treaty known as the Vienna Convention on Consular Relations.

In an interview televised in Mexico on Tuesday, the governor of Morelos decried the "arrogance" and "racism" of Texas' legal system and said the Tamayo case "violated a fundamental principle, which is consular assistance."

Graco Ramirez, a member of Mexico's Democratic Revolution Party, or PRD, said that although he didn't know whether Tamayo was guilty, "what is certain is that due process wasn't given, and when such due process isn't granted as a judicial principle, clearly there's no certainty about what's being judged."

The United Nations International Court of Justice, also known as the World Court, ordered the U.S. 10 years ago to reconsider the convictions of 51 Mexicans, including Tamayo, who had been sent to death row without being told of their consular rights. Two of the 51 have since been executed, both in Texas.

In 2005, President George W. Bush ordered Texas and other states to review the 51 convictions. But Texas' then-solicitor general, Ted Cruz, now a senator, persuaded the U.S. Supreme Court to rule that the president had no authority to order state courts to defer to the World Court.

The 32 states with capital punishment have executed 28 foreign nationals since 1976, according to the Washington-based Death Penalty Information Center, which opposes the practice.

"Mexico typically intervenes in these cases, capital cases, even before the trials occur so they often don't result in a death sentence anymore. It might well have made a difference in Tamayo's case," said the center's executive director, Richard Dieter.

Mexican officials have petitioned the U.S. on Tamayo's behalf, including Foreign Secretary Jose Antonio Meade and Ambassador to the U.S. Eduardo Medina Mora.

Last week, Mark White, a former Texas governor and state attorney general, joined the effort.

"I personally support capital punishment. But this case is not about whether we support or oppose the death penalty. It's about fairness and having the courts hear all the key facts. In Tamayo's case, a court review could have made a real difference," White, a Democrat, wrote in the Austin American-Statesman.

Kerry has urged Texas to reconsider.

"I have no reason to doubt the facts of Mr. Tamayo's conviction, and as a former prosecutor, I have no sympathy for anyone who would murder a police officer," he wrote in September, adding that he was concerned that Texas' handling of the case could affect the way Americans are treated overseas.

Kerry shared a letter he received from Medina Mora. "This issue has become and could continue to be a significant irritant in the relations between our two countries," the ambassador wrote.