Democratic Senator Bob Menendez called the account “alarming & unacceptable if true”, tweeting that “we must get to the bottom of this” at a Senate hearing set for Thursday to review the 2015 Trafficking in Persons report.

A Reuters examination, based on interviews with more than a dozen people in Washington and foreign capitals, showed that the State Department office set up to independently grade global efforts to fight human trafficking was repeatedly overruled by senior diplomats and pressured into inflating assessments of 14 countries in this year’s report.

Among the countries that received higher rankings than recommended by the Office to Monitor and Combat Trafficking in Persons were Malaysia, Cuba, China, India, Uzbekistan and Mexico, the sources said.

“‎It’s shameful that President Obama allowed a bunch of political hacks to alter the administration’s human trafficking report to the benefit of perennial violators like Cuba and Malaysia,” said U.S. Senator Marco Rubio, who is also a Republican presidential candidate.

Rubio called it a “dangerous precedent”. He sits along with Menendez on the Senate Foreign Relations Committee, which this week will question Sarah Sewall, who oversees the anti-trafficking office as Undersecretary of State for Civilian Security, Democracy and Human Rights.

Former Florida governor Jeb Bush, another Republican presidential candidate, also weighed in. “Obama and State Dept should be ashamed of their purely political manipulation of Cuba's human trafficking issues,” he tweeted.

Analysts in the anti-trafficking office, or J/TIP, as it is known within the U.S. government, disagreed with U.S. diplomatic bureaus on ratings for 17 countries, the sources said.

The analysts, who are specialists in assessing efforts to combat modern slavery - such as the illegal trade in humans for forced labor or prostitution - won only three of those disputes, the worst ratio in the 15-year history of the unit, in the report published on July 27, according to the sources.

Cuba, Malaysia and Uzbekistan were upgraded, despite J/TIP’s objections, from the lowest ranking in the report that publicly shames the world’s worst offenders in human trafficking.

The Malaysian upgrade could smooth the way for an ambitious proposed U.S.-led free-trade deal with the Southeast Asian nation and 11 other countries.

The number of rejected recommendations suggests a degree of intervention not previously seen by top State Department diplomats in a report that can lead to sanctions.

Human rights groups and some former State Department officials have expressed concern that such unearned higher grades undermine the credibility of the annual report.