Washington (CNN)The Justice Department is preparing to bring criminal corruption charges against New Jersey Sen. Robert Menendez, a 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 has learned exclusively. An announcement could come within weeks. Prosecutors are under pressure in part because of the statute of limitation on some of the allegations.

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.

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.

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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 has denied any wrongdoing in his ties to Melgen.

« As we have said before, we believe all of Senator’s actions have been appropriate and lawful and the facts will ultimately confirm that, » Menendez spokesperson Tricia Enright said in a statement Friday. « Any actions taken by Senator Menendez or his office have been to appropriately address public policy issues and not for any other reason. »

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.

READ: Menendez: White House’s least favorite Dem

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. »