Two years ago, Dr. Melgen, despite an apparent lack of experience in border security issues, bought an ownership interest in a company that had a long-dormant contract with the Dominican Republic to provide port security. Mr. Menendez, who is chairman of the Senate subcommittee that holds sway over the Dominican Republic, subsequently urged officials in the State and Commerce Departments to intervene so the contract would be enforced, at an estimated value of $500 million…

In Florida political circles, one Miami Democrat explained, it is understood that anyone seeking a federal appointment that requires Mr. Menendez’s blessing should first get Dr. Melgen’s backing.

“If you needed Bob, you had to see Melgen,” said the Democrat, who insisted on anonymity for fear of upsetting party leaders. “Everybody in Miami knew that.”

In addition to the security company, in 2011 Dr. Melgen founded a Latino-oriented news Web site, Voxxi, which has also emphasized the need for the port security deal and glowingly depicted Mr. Menendez as a giant among Latinos. A former employee, who spoke on the condition of anonymity out of fear of career damage, said Dr. Melgen would often intervene in the coverage, requiring editors to play down achievements by Republicans like Senator Marco Rubio of Florida and replace it with praise for Mr. Menendez.