However he maintained that Justice Department prosecutors were "tricked" into starting an investigation because of a "political motive to silence" the senator. "They are dead wrong and this is not how my career is going to end."
Mr Menendez, one of the most influential Hispanic American lawmakers in Congress, has previously staunchly defended himself, denying the allegations to reporters on March 6 saying he had "conducted myself appropriately" and not broken the law.
If he did so, however, New Jersey's Republican governor, Chris Christie, would likely appoint a Republican replacement to serve until a special election.
That would bring the number of Republicans in the Senate to 55, one seat closer to the 60-seat super majority needed to overcome Democrats' procedural roadblocks and advance legislation.
The 61-year-old senator has acknowledged that he and Mr Melgen have been close friends for more than two decades, celebrating holidays, family weddings, funerals and birthdays together.
Mr Menendez was invited to Melgen's villa in the Dominican Republic on several occasions, and had to repay tens of thousands of dollars for several trips on the doctor's plane after admitting to a mix-up with air fares from his 2012 re-election campaign.
Menendez is also accused of intervening in 2009 and 2012 with the agency that oversees the Medicare senior health program following a ruling that Mr Melgen had overbilled the US government by nearly $9 million, according to the Politico news website.
Mr Menendez is a Cuban-American and one of the most senior Hispanic politicians in the United States who has played a central role in lobbying for immigration reform. He was first elected to the Senate in 2005 and was re-elected in 2012.
The last senator to be indicted, Alaska Republican Ted Stevens, did not resign after he was charged with failing to report gifts and services from an oil company in 2008. Mr Stevens lost his race for re-election that year, and his conviction was dismissed in 2009.
New Jersey has a long history of corruptions scandals, including the 1981 bribery conviction of the late Harrison Williams, a New Jersey senator who occupied the same seat as Mr Menendez, and was sentenced to three years in a Federal penitentiary for his crimes.
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Top Democrat Senator Robert Menendez indicted on corruption charges