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The claim was included in an indictment that was obtained against Menendez by the Department of Justice on Wednesday.
The Justice Department said the politician and his staff aided the three women, who are all foreigners, so they could enter the U.S. to visit Melgen, a successful eye surgeon in South Florida. None of the women were named in the indictment.
Melgen is reportedly still married to his wife, Flor. However, the indictment chronicles his alleged extramarital dalliances with women from Brazil, the Dominican Republic and Ukraine that occurred from 2007 to 2008.
According to federal prosecutors, Melgen sought assistance from Menendez for his Brazilian love interest, who was identified in the indictment as Girlfriend #1, in 2008.
Prosecutors described Girlfriend #1 as an actress, model, and lawyer, who needed help to secure a student visa in order to pursue an LLM degree in the U.S. at the University of Miami.
In July 2008, Menendez's senior policy adviser allegedly emailed the Deputy Assistant Secretary at Visa Services for the Department of State to "encourage careful consideration" of Girlfriend #1's visa application. The unidentified woman was granted a visa and Melgen's non-profit organization funded part of her tuition.
"Girlfriend 1 met Menendez several times while with Melgen in New York, New Jersey, Florida, Spain and the Dominican Republic," the indictment claimed.
At around the same time Menendez was helping Melgen with Girlfriend #1's scholastic endeavors, the doctor also allegedly sought Menendez's help on behalf of Girlfriend #2, a Dominican who worked as a model.
Prosecutors said Melgen began a relationship with the Dominican woman in 2005. In October 2008, Melgen allegedly wanted to get a travel visa for Girlfriend #2 and her sister so they could visit him in West Palm Beach, Florida.
Despite a letter of recommendation from Menendez to the Consul General of the U.S. Embassy in the Dominican Republic, the indictment said those visa applications were initially denied.
The indictment said the State Department expressed doubts about Girlfriend #2 and her sister, aged 22 and 18, since they had "no solvency of their own" and the agency was "not fully convinced of motives for travel."
When Menendez heard of the news, he allegedly emailed a member of his staff to put him in contact with either the country's ambassador or a contact at the State Department.
After intervention from Menendez's office, prosectuors said the pair were then called back for a second interview at the U.S. Embassy in December and their visa applications were ultimately approved.
"In my view this is ONLY DUE to the fact RM intervened. I've told RM," a Menendez staffer told another staffer in an email, according to the indictment.
In 2007, Menendez also allegedly worked on behalf of another Melgen flame, who was identified in the indictment as Girlfriend #3, a Ukrainian who worked as a model and actress.
According to prosecutors, Girlfriend #3 lived in Spain and wanted to visit Florida but needed a tourist visa to enter the U.S.
In February 2007, Menendez's Chief of Staff allegedly intervened to facilitate the visa process. The indictment said the woman obtained the visa and during her 2007 visit to Florida, she dined with Melgen and Menendez at Azul, a restaurant at the Mandarin Hotel in Miami.
"Melgen introduced Menendez to Girlfriend 3 as the man who helped Girlfriend 3 with her visa," the indictment stated.
Menendez's office has not yet commented on the charges but the senator has previously denied any wrongdoing.