Criticizing the conviction last year, Mr. Freeh said the decision against Mr. Popoviciu was “not supported by either the facts or the law.”

In a statement to The New York Times in response to questions about Mr. Giuliani’s letter, a representative of the United States Embassy in Bucharest, the Romanian capital, said that the government did not comment on the views or conclusions of private American citizens.

The representative also pointed to a June 28 statement issued by the embassy, along with the embassies of 11 other countries, which highlighted Romania’s “considerable progress” in combating corruption and in building an effective rule of law. The statement, which came at a time when contentious alterations to the criminal code were moving through Parliament, also called on all parties involved to “avoid changes that would weaken the rule of law or Romania’s ability to fight crime or corruption.”

Starting in January 2017, the government pushed through measures that many fear will weaken the rule of law. Critics of the anticorruption drive accuse those involved of using it for political purposes and of relying heavily on the use of court-approved wiretaps, while its defenders say that the effort was under fire because of its success in going after vested interests and powerful individuals.

Last month, Laura Codruta Kovesi, the chief prosecutor of the Romanian National Anticorruption Directorate, was fired after accusations of overstepping her authority and damaging Romania’s image abroad. The tensions have led to major protests, the most recent of which, on Aug. 10, resulted in violent clashes in Bucharest that left more than 400 people wounded.

Mr. Giuliani’s letter was well received by some in Romania. Liviu Dragnea, the leader of the leftist Social Democratic Party, the biggest party in the Romanian Parliament, wrote on Facebook that Mr. Giuliani “puts his finger on the wound.”

Mr. Dragnea was sentenced in June to three-and-a-half years in prison after Romania’s highest court found him guilty of abuse of office related to the fictitious hiring of two of his party’s employees at a public institution. He is appealing the verdict. In a separate case in 2015, Mr. Dragnea was convicted of electoral fraud and received a suspended sentence.