Presidential contender and Sen. Marco Rubio, R-Fla., is fighting members of his own party in the Senate as it moves toward a possible vote on the nomination of State Department official Roberta Jacobson to be the next U.S. ambassador to Mexico.

Jacobson is known for leading negotiations that helped the U.S. and Cuba take steps toward normalized relations, a move that many Republicans and Democrats opposed, especially Cuban-Americans like Rubio.

On Tuesday, Rubio released a lengthy statement saying Jacobson oversaw the division within the State Department that downgraded Cuba as a country involved in human trafficking. He said that sends a "chilling signal about the integrity of U.S. human trafficking assessments of a country that investigations have shown to be one of the top destinations in the Americas for sex tourism."

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Rubio, who leads the Washington Examiner's power rankings, also said Jacobson and the Obama administration have so far failed to push Cuba on human rights effectively, and said her inability to provide clear answers to his questions makes her untrustworthy.

"We need an ambassador in Mexico City that has the trust of Congress for this important post," he said. "I do not believe that Ms. Jacobson is that person and will oppose her confirmation."

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