NEWARK, N.J. (Reuters) - U.S. Senator Bob Menendez abused his power to help a wealthy benefactor in a years-long bribery scheme that netted the New Jersey Democrat lavish gifts and major campaign donations, federal prosecutors told a jury on Wednesday.

“This case is about a corrupt politician who sold his Senate office for a life of luxury he couldn’t afford,” the lead prosecutor, Peter Koski, said at the start of Menendez’s corruption trial in Newark.

Menendez’s lawyer, Abbe Lowell, said prosecutors had twisted a close, brother-like relationship into something illicit.

“Acting out of friendship is not improper, it is not corrupt, and it is certainly not a crime,” he said.

Menendez is accused of intervening with federal officials to help his co-defendant Salomon Melgen, a Florida ophthalmologist who gave the senator private flights, stays at a Caribbean villa and hundreds of thousands of dollars in campaign money.

The case comes at a crucial time in Washington, D.C., where Republicans hold a razor-thin 52-48 edge in the Senate. If Menendez is convicted and either resigns or is expelled by his colleagues before January, his replacement would be named by New Jersey Governor Chris Christie, a Republican.

A group of supporters cheered Menendez, 63, as he entered court on Wednesday morning. The senator told reporters he had “never backed away from a fight.”

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He is running for a third six-year term in 2018 despite the charges.

Koski told jurors Menendez used his influence to help Melgen, securing visas for Melgen’s foreign girlfriends and pressuring the U.S. State Department to resolve a contract dispute between a company Melgen owned and the Dominican Republic.

After Medicare, the federal insurance plan for the elderly and disabled, concluded Melgen had overbilled it by $8.9 million, Menendez asked officials to change the reimbursement policy, Koski said.

Melgen was convicted earlier this year of perpetrating a massive Medicare fraud. He has yet to be sentenced.

Koski also urged jurors to pay attention to the timing of Melgen’s gifts. In April 2012, for instance, Menendez met with an assistant secretary of state regarding the Dominican Republic dispute; Melgen gave Menendez $80,000 on the same day.

“Robert Menendez was Salomon Melgen’s personal United States senator,” Koski said.

Lowell said Menendez only acted at Melgen’s suggestion after researching the issue with his staff and concluding it was sound policy.

He also accused prosecutors of cherry-picking a handful of examples amidst countless similar interventions that Menendez has made over the years when matters were brought to his attention.

The trial is expected to last up to eight weeks.