U.S. Sen. Robert Menendez of New Jersey has been indicted on federal corruption charges in connection with an ongoing investigation into his business dealings with a Florida doctor, authorities said Wednesday.

The indictment charges the Democrat, who has held his seat since 2006, with 14 counts, including bribery and conspiracy, over his ties to Salomon Melgen, a wealthy Florida optometrist and the politician's longtime friend. He is expected to appear in federal court in Newark Thursday. 

At a news conference Wednesday, Menendez said he was "outraged" by the prosecutors at the Justice Department for being "tricked into starting this investigation three years ago with false allegations by those who have a political motive to silence me." 

"I'm angry and ready to fight because today contradicts my public service career and my entire life," he added. 

The indictment states that Menendez accepted nearly $1 million worth of "lavish gifts" and campaign contributions from Melgen in exchange for using the power of his Senate office to influence the outcome of ongoing contractual and Medicare billing disputes worth tens of millions of dollars to Melgen.

Melgen was also seeking Menendez's support of the visa applications of several of Melgen's girlfriends, according to the indictment. 

Menendez temporarily stepped down Wednesday as ranking member of the Senate Foreign Relations Committee, where he was the top Democrat on the influential committee, NBC News reported.

In a letter to Senate Democratic Leader Harry Reid, Menendez wrote: "While there is no caucus rule that dictates that I do so, I believe it is in the best interests of the Committee, my colleagues, and the Senate which is why I have chosen to do so."

He added that he wants to return to the post "upon the successful resolution of the allegations before me," and that he expects to retain his membership and seniority in the committee.

Melgen was also charged in the case; he could not immediately be reached Wednesday afternoon.

Previously, both Menendez and the doctor had denied wrongdoing. Representatives for the U.S. Department of Justice and the FBI declined comment.

The indictment from a grand jury in Menendez's home state was the latest development in a federal investigation that came into public view when federal authorities raided Melgen's medical offices two years ago. The investigation focused on whether the senator had improperly advocated on Melgen's behalf, including by intervening in a Medicare billing dispute.

Menendez has acknowledged that he flew multiple times on Melgen's private jet to the Dominican Republic and initially failed to properly pay for the trips. Menendez in 2013 agreed to reimburse Melgen $58,500 for the full cost of two flights.

The senator's office later disclosed another flight, from Florida to New Jersey in 2011, and said Menendez had repaid Melgen $11,250 for it.

Last year, Menendez disclosed that his campaign accounts had paid a law firm $250,000 for legal costs related to investigations by the Justice Department and the Senate Ethics Committee of his ties to Melgen.

Menendez has acknowledged taking actions that could benefit Melgen, among them contacting U.S. health agencies to ask about billing practices and policies. But the lawmaker has said he did nothing wrong and that he and Melgen have been friends for decades.

"We celebrated holidays together," he once told reporters. "We have been there for family weddings and sad times like funerals and have given each other birthday, holiday and wedding presents, just as friends do."

At the news conference Wednesday, Menendez said: "I'm angry because prosecutors at the Justice Department don't know the difference between friendship and corruption, and have chosen to twist my duties as a senator and my friendship into something that is improper. They are dead wrong and I am confident they will be proven so." 

Melgen came under renewed scrutiny when government data last year showed he had received more in Medicare reimbursements in 2012 than any other doctor in the country.

The indictment alleges that Menendez engaged in three efforts to use his Senate office and staff to advocate on behalf of Melgen's personal and financial interests, according to a press release from the Justice Department:

First, Menendez allegedly pressured executive agencies in connection with a conflict between Melgen and the government of the Dominican Republic relating to a disputed contract that Melgen purchased to provide exclusive screening of containers coming through Dominican ports.

Second, Menendez allegedly advocated on behalf of Melgen in connection with a Medicare billing dispute worth approximately $8.9 million to Melgen.

Third, Menendez allegedly took active steps to support the tourist and student visa applications of three of Melgen’s girlfriends, as well as the visa application of the younger sister of one of Melgen’s girlfriends.

Menendez lobbied on behalf Melgen's interests all the way up to the highest levels of U.S. government, according to the Justice Department, meeting with a U.S. cabinet secretary, contacting a U.S. ambassador, meeting with other senior agency officials and executives, and soliciting other U.S. senators. 

Menendez becomes the first sitting U.S. senator to face indictment since then-Sen. Ted Stevens, R-Alaska, in 2008. Stevens was found guilty of concealing gifts from donors on financial disclosure statements, but the Justice Department later dropped the case after admitting that prosecutors failed to turn over evidence that would have been favorable to his defense.

Menendez joined the Senate after serving more than a decade in the House of Representatives. A lawyer and former mayor of Union City, he also served in the New Jersey General Assembly and state Senate.

Even while under federal investigation, he has used his leadership position on the Senate Foreign Relations Committee to criticize negotiations between President Obama's administration and Iran on its nuclear program and has been outspoken in opposition to normalizing relations with Cuba.

The indictment comes as the confirmation of Loretta Lynch, the U.S. Attorney for the Eastern District of New York nominated by Obama as U.S. Attorney General, hangs in the balance.

Menendez has not yet said how he would vote. Washington insiders tell NBC 4 New York either a vote of yes from Menendez or Illinois Republican Sen. Mark Kirk is needed to get Lynch the 50 votes she needs to be confirmed.

Separate from Wednesday's indictment, investigators have said they are looking into the senator’s relationship with two fugitive Ecuadorean bankers living in Florida. 

New Jersey's junior U.S. Sen. Cory Booker (D-NJ) expressed support for Menendez in a statement Wednesday, calling him an "invaluable resource and a mentor" since he arrived in the Senate.

"I won't waver in my commitment to stand alongside my senior Senator to serve our great state," he said.