Updated at 8:15 p.m. ET
Menendez told reporters during a Friday night press conference — during which he read from a statement and declined to take questions — that he always had behaved appropriately while in office.
"Let me be very clear — very clear: I have always conducted myself appropriately and in accordance with the law," the senator said. "Every action that I and my office have taken for the last 23 years that I have been privileged to be in the United States Congress has been based on pursuing the best policies for the people of New Jersey and of this entire country."
He also defended his record and vowed not to resign.
"I fight for these issues and for the people of our country every single day," Menendez said. "That's who I am, and I am not going anywhere."
Updated at 3:17 p.m. ET
The Justice Department is planning to bring corruption charges against Sen. Robert Menendez, D-N.J., alleging that he did political favors for a friend and donor, NPR's Carrie Johnson has confirmed.
A person familiar with the case tells Carrie that a decision has been made to go forward with a prosecution.
"It is not clear how long it will take for actual criminal charges to emerge," Carrie says.
The case is being handled by the Justice Department's Public Integrity Section in Washington, she adds.
Tricia Enright, a spokeswoman for Menendez, said in a statement, "As we have said before, we believe all of the senator's actions have been appropriate and lawful and the facts will ultimately confirm that. Any actions taken by Senator Menendez or his office have been to appropriately address public policy issues and not for any other reason."
News of the planned charges was first reported by CNN. The case against Menendez stems from his relationship with a Florida eye doctor named Salomon Melgen.
Menendez, who has emerged as a recent critic of the Obama administration's policy on Iran and Cuba, came under scrutiny two years ago. As NPR's Peter Overby reported at the time, "Melgen ... has been a longtime and generous supporter. [In 2012], his medical practice gave $700,000 to a Democratic superPAC, which spent nearly $600,000 to help Menendez in the November election."
As Peter noted in his reporting, Menendez pressed two State Department officials during a hearing about an American company that was providing port security in the Dominican Republic. But, Menendez said during the hearing, local officials "don't want to live by that contract." And the senator said the U.S. needed to side with the company, ICSSI, not the Dominican government.
What he didn't say was that ICSSI was partially owned by Melgen.
And twice in 2009, Peter reported, "Menendez went to Medicare on Melgen's behalf after health care officials alleged the doctor had overbilled by nearly $9 million. ... Menendez has also admitted that he failed to disclose two trips on Melgen's private jet — flights to a Dominican Republic resort community where Melgen has a house."
Menendez later reimbursed Melgen for the flights.
According to the Center for Responsive Politics, Menendez had an estimated net worth of $448,002 in 2013.