Washington (CNN) The Justice Department is preparing to bring criminal corruption charges against Sen. Robert Menendez, a New Jersey Democrat, alleging he used his Senate office to push the business interests of a Democratic donor and friend in exchange for gifts.

People briefed on the case say Attorney General Eric Holder has signed off on prosecutors' request to proceed with charges, CNN first reported. An announcement could come within weeks. Prosecutors are under pressure in part because of the statute of limitation on some of the allegations.

Menendez told reporters Friday night he has "always conducted myself appropriately and in accordance with the law."

He added: "And I am not going anywhere."

Menendez said that because of an ongoing investigation he couldn't answer questions at the news conference, held in Newark.

The case could pose a high-profile test of the Justice Department's ability to prosecute sitting lawmakers, having already spawned a legal battle over whether key evidence the government has gathered is protected by the Constitution's Speech and Debate clause

The FBI and prosecutors from the Justice Department's public integrity section, have pursued a variety of allegations against Menendez, who has called the probe part of "smear campaign" against him.

Photos: Politicians in hot water Photos: Politicians in hot water U.S. Rep. Chaka Fattah was convicted on federal corruption charges on Tuesday, June 21. The Philadelphia Democrat was tied to a host of campaign finance schemes, according to the Department of Justice. Hide Caption 1 of 12 Photos: Politicians in hot water Former Speaker of the House Dennis Hastert was sentenced to 15 months in prison and ordered to pay $250,000 to a victims' fund in April after a hush-money case revealed he was being accused of sexually abusing young boys as a teacher in Illinois. Hide Caption 2 of 12 Photos: Politicians in hot water Former Virginia Gov. Bob McDonnell arrives at his corruption trial in Richmond, Virginia, in September 2015. A jury convicted McDonnell and his wife, Maureen, derailing the political ambitions of the one-time rising star in the Republican Party. McDonnell, who was sentenced to two years in prison, has asked the Supreme Court to reverse his conviction. The high court heard his challenge in April. Hide Caption 3 of 12 Photos: Politicians in hot water U.S. Sen. Robert Menendez, a New Jersey Democrat, was indicted on corruption charges in April 2015. Federal prosecutors have accused Menendez of using his Senate office to push the business interests of a friend and donor in exchange for gifts. The senator has pleaded not guilty to the charges and vehemently asserts his innocence. Hide Caption 4 of 12 Photos: Politicians in hot water U.S. Rep. Vance McAllister asked for forgiveness from God, his family and his constituents after a newspaper published what it said was surveillance video showing the married Louisiana Republican making out with a female staffer. His term ended in 2015. Hide Caption 5 of 12 Photos: Politicians in hot water Former U.S. Rep. Jesse Jackson Jr. and his wife, Sandra, arrive at federal court in Washington for sentencing in August 2013. Jackson, a Democrat from Illinois, was sentenced to 30 months in prison for improper use of campaign funds, while his wife got 12 months for filing false tax returns. Hide Caption 6 of 12 Photos: Politicians in hot water In 2012, former Illinois Gov. Rod Blagojevich was sentenced to 14 years in prison after being convicted of 18 criminal counts, including trying to sell the appointment to fill the U.S. Senate seat vacated by Barack Obama. Hide Caption 7 of 12 Photos: Politicians in hot water Lewis "Scooter" Libby, former chief of staff for Vice President Dick Cheney, was convicted in 2007 of obstructing a federal investigation into who revealed the identity of CIA agent Valerie Plame Wilson. He was sentenced to 30 months in prison and fined $250,000, but former President George W. Bush commuted his sentence. Hide Caption 8 of 12 Photos: Politicians in hot water Former U.S. Rep. William Jefferson was sentenced to 13 years in prison in 2009 after being convicted of 11 counts of corruption related to using his office to solicit bribes. The Louisiana Democrat was also ordered to forfeit $470,000. Hide Caption 9 of 12 Photos: Politicians in hot water Former U.S. Rep. Bob Ney, a Republican from Ohio, was sentenced to 30 months in prison in 2007 after being convicted of conspiracy to commit fraud and making false statements to investigators. Hide Caption 10 of 12 Photos: Politicians in hot water U.S. Rep. James Traficant Jr., D-Ohio, spent seven years in prison after being convicted of bribery and corruption and tax evasion charges in 2002. Hide Caption 11 of 12 Photos: Politicians in hot water Former U.S. Rep. Randy "Duke" Cunningham, R-California, was sentenced to eight years in prison in 2006 after he was convicted of collecting $2.4 million in homes, yachts, antique furnishings and other bribes on a scale unparalleled in the history of Congress. Hide Caption 12 of 12

The government's case centers on Menendez's relationship with Salomon Melgen, a Florida ophthalmologist who the senator has called a friend and political supporter. Melgen and his family have been generous donors to the senator and various committees the senator is associated with.

Investigators have focused in part on plane trips Menendez took in 2010 to the Dominican Republic as a guest of Melgen. In 2013, after word of the federal investigation became public, Menendez paid back Melgen $58,000 for the 2010 plane trips calling his failure to properly disclose the flights an "oversight."

Menendez said he has been friends with Melgen for more than 20 years and the two families have spent holidays and other special occasions together.

One of the highest ranking Hispanic members of Congress, Menendez is a former chairman of the Senate Foreign Relations Committee. He has become one of the Obama administration's most vocal Democratic opponents on two key foreign policy matters -- President Obama's decision to ease the trade embargo against Cuba and also his effort to engage direct negotiations with Iran over that country's nuclear program.

Menendez advocated on Melgen's behalf with federal Medicare administrators who accused Melgen of overbilling the government's healthcare program, according to court documents and people briefed on the probe. Melgen was among the top recipients of Medicare reimbursements in recent years, during a time when he was also a major Democratic donor. Melgen's attorneys have denied any wrongdoing.

Prosecutors also are focusing on whether Menendez broke the law in advocating for Melgen's business interest in a Dominican Republic government contract for a port screening equipment. The U.S. Customs and Border Protection agency, at the time, considered donating port screening equipment to the Dominican Republic, which would have hurt the contract of Melgen-controlled company.

Menendez, now serving his second full term as senator, led the Democratic Senatorial Campaign Committee from 2009-2011.

During a Senate subcommittee hearing in 2012 , Menendez didn't mention ICSSI, Melgen's company, by name, but he did press Deputy Assistant Secretary of State Matthew Rooney about an unnamed company who had a contract to X-ray cargo that went through all Dominican ports -- a contract that, he said, Dominican authorities "don't want to live by."

"If those countries can get away with that, they will," the senator said. "And that puts American companies at a tremendous disadvantage."

Menendez's office said at the time the senator's interest was based on his efforts to combat narcotrafficking in the region.

Other lines of inquiry against Menendez had included allegations he solicited prostitutes in the Dominican Republic , and that he violated the law helping win permanent U.S. residency for two Ecuadorian banking magnates, the Isaias brothers. The prostitution allegations collapsed after the purported prostitutes recanted their story, and the FBI didn't find evidence of wrongdoing in the Isaias matter, according to people briefed on the probe.

The FBI probe has already spawned a legal battle between the government, Menendez and his former aides. Last week, the Third Circuit Court of Appeals briefly posted, apparently by accident, documents detailing legal efforts to block certain evidence and testimony the government wants to use.

The documents were quickly put back under seal, but not before a reporter with the New Jersey Law Journal secured a copy and later published a story.

According to the documents, Menendez's lawyers have sought to claim emails and testimony from aides is protected by the constitutional protections given to members of Congress in carrying out their duties. The speech and debate clause prohibits questioning of members of Congress about "legislative acts or the motivation for legislative acts."

The fight centers in part on the Justice Department's attempt to compel testimony from Menendez's aides, some of whom have refused to answer questions to a grand jury.

According to the documents, the government wants to question aides about a series of 2012 calls and meetings on Melgen's fight with the Center for Medicare and Medicaid Services, a federal agency. Among these is a meeting among Mendedez, Sen. Harry Reid and then Health and Human Services Secretary Kathleen Sebelius. The government is also pushing to use emails between Menendez's office and a CBP official about the Dominican ports issue.

A federal district court ruled in favor of the government to compel testimony from Menendez's aides, but the appeals court reversed the ruling and ordered a hearing.

"The parties primarily dispute the legislative character of Senator Menendez's two conversations with [then acting CMS administrator Marilyn] Tavenner and his meeting with Secretary Sebelius," the appeals court said. "These communications are not manifestly legislative acts because they are informal communications with executive branch officials, one of whom was at the time a presidential nominee whose nomination was pending before the United States senate."