The federal bribery case against Democratic Sen. Robert Menendez of New Jersey marks the first time large-scale super PAC donations have figured prominently as evidence of a political corruption scheme, renewing questions about how truly independently such groups operate.

The 22-count indictment against Menendez and wealthy Florida ophthalmologist Salomon Melgen hinges in part on $600,000 that Melgen gave to the Senate Majority PAC — a Democratic super PAC — earmarked to support the senator’s 2012 reelection.

Senate Majority PAC officials have not been accused of any wrongdoing. But the Justice Department argued in the court filing that the donations were among the things of value Melgen offered Menendez so the senator would use his position to help get the donor’s girlfriends visas to enter the country and to influence government officials to help Melgen’s businesses.

The case illustrates how super PACs — which can accept unlimited donations and are supposed to be walled off from the candidates they support — are viewed by donors as vehicles to ingratiate themselves with politicians.

“You just see that everything that goes on behind the scenes confirms people’s worst fears: that someone can swear that there’s no quid pro quo and no deals, but it’s very hard to know if that’s true, and it depends in your confidence in politicians to tell the truth,” said Richard L. Hasen, a law professor at the University of California at Irvine who studies campaign finance regulation.

Critics of the current era of unfettered political spending said the Menendez case shows that it is easier for the very rich to unduly influence elected officials through groups that accept unlimited amounts of money.

“With so much money at stake, it’s not surprising to find senators and representatives paying special attention to the desires of their donors,” Miles Rapoport, president of Common Cause, said in a statement.

But advocates of loosening campaign finance restrictions said that just because Melgen may have thought he could influence Menendez through super PAC donations does not mean the senator actually was compromised, even if he knew about the donations.

“Gratitude is not corruption,” said conservative election law attorney James Bopp Jr. “Was there an exchange that is related to the super PAC? And if there was, then it would be corrupting. . . . But there is nothing inherently corrupting in somebody who wants favors or likes somebody giving money to a super PAC.”

David Keating, president of the Center for Competitive Politics, which supports less campaign finance regulation, said the alleged corruption was not caused by the avenue that Melgen used.

“Corrupt people do corrupt things,” he said. “The answer isn’t to eliminate people’s free-speech rights any more than it would be to eliminate luxury condos in the Caribbean.”

The Supreme Court has held since the 1976 Buckley v. Valeo case that independent political spending does not corrupt elected officials. That was the argument that underpinned the landmark 2010 Citizens United v. Federal Election Commission case, in which the justices said that corporations and unions could spend unlimited sums of money on political activity.

“We now conclude that independent expenditures, including those made by corporations, do not give rise to corruption or the appearance of corruption,” Justice Anthony M. Kennedy wrote in the decision.

“There is only scant evidence that independent expenditures even ingratiate,” he added. “Ingratiation and access, in any event, are not corruption.”

Hasen said that the Menendez case shows the court’s flawed logic. “It’s impossible to ensure independence, because a deal like this could have been kept secret from the super PAC,” he said.

One of the key issues in the case will be whether Menendez played a role in directing Melgen to make the super PAC contributions. Under campaign finance rules, a federal officeholder may only solicit donations of up to $5,000 for a super PAC.

Melgen donated $600,000 to Senate Majority PAC from the corporate treasury of his company, Vitreo-Retinal Consultants, in 2012. That year, the super PAC spent $582,000 on behalf of Menendez’s reelection, according to data from the Center for Responsive Politics. The senator ended up beating Republican Joe Kyrillos by more than 19 percentage points.

In a statement, Stephanie Potter, executive director of Senate Majority PAC, said that Melgen was “one of hundreds of donors supporting Senate Majority PAC’s mission of electing Democrats to the US Senate.”

“Nothing in today’s indictment casts doubt on the lawfulness of Senate Majority PAC, its actions, or its mission,” she said. “The FEC has specifically held that super PACs may accept contributions earmarked for particular races — and indeed, they routinely do.”

It remains to be seen what evidence prosecutors have linking Menendez to the super PAC donations. But four bribery counts in the indictment are based on the donations to the group, alleging that the senator “sought and received” $600,000 from Melgen for Senate Majority PAC in exchange for Menendez’s efforts to help Melgen resolve a Medicare billing dispute.

The first of two $300,000 checks Melgen wrote to Senate Majority PAC was issued on the same day in June 2012 that the ophthalmologist attended Menendez’s annual fundraising event in New Jersey, the indictment says.

Melgen gave the check to a close friend of Menendez, who then sent it via FedEx to a fundraiser for Senate Majority PAC, according to the court filing. After receiving it, the fundraiser noted in an e-mail that $300,000 had arrived “earmarked for New Jersey.”

Melgen wrote another $300,000 check to the super PAC on Oct. 12, 2012 — a week before sending Menendez an e-mail with information about a Medicare audit of his company that he was trying to fend off, according to court documents.

On the same day, Melgen also e-mailed a Senate Majority PAC fundraiser with the information about his Medicare case, the indictment says. The fundraiser offered to share the information with “Senator 3,” who is identifiable from the court filing as then-Senate Majority Leader Harry M. Reid (D-Nev.), for whom the fundraiser had previously worked.

Potter said that Melgen and the fundraiser, who no longer works for the super PAC, are personal friends. Senate Majority PAC officials were not aware of “the full extent of their communication” until reading about it in the indictment, she said.

“Senate Majority PAC carefully adheres to the letter and the spirit of the law, and we take pride in that,” Potter said. “We take our separation from official government action very seriously and do not advocate legislation before the Senate.”

Paul Kane contributed to this report.