Bannon's shady 2016 dealings still under investigation -- but his billionaire backers may have already abandoned him

Voting rights watchdogs are hopeful that a recently closed investigation could help renew interest in a long-stymied probe of Steve Bannon and his former financial backers.

The Federal Election Commission has concluded its investigation into possibly illegal coordination with Cambridge Analytica's foreign employees in the 2014 and 2016 elections, after coming up with "reason to believe" that occurred, but the commission's Republican commissioners voted to exclude possible violations by Donald Trump's campaign -- which involved many of the same players that landed under scrutiny, reported The Daily Beast.

"Part of what's so striking is the revolving door between the Trump campaign, Bannon's super PAC, and Cambridge Analytica, in some cases all at the same time," Brendan Fischer, director of reform at the nonpartisan Campaign Legal Center.

The CLC filed the initial complaint five years ago, but a closely related complaint involving Steve Bannon and his billionaire backers, hedge fund billionaire John Mercer and his daughter Rebekah Mercer, remains open -- and the recently closed probe offers a glimpse into the other and suggests the right-wing megadonor may have turned on Bannon.

WATCH: Bannon attorney clashes with CNN's Keilar over executive privilege

"The response from Mercer's attorney was that Bannon was running the show, so the idea that he'd also been running both the super PAC and the Trump campaign, and that both of them were contracting with the firm Bannon had also been running just boggles the mind," Fischer said.

"These are not novel issues," he added. "It's anticipated in the law that a shared vendor might act illegally as a conduit between campaigns. But this is something we haven't really seen before."

Bannon, who briefly served in Trump's White House, ran U.S. operations for Cambridge Analytica while overseeing a pro-Trump PAC, and then joined the 2016 campaign, and the campaign and the PAC paid the tech firm nearly $11 million total, in possible violation of a law against campaigns and outside groups to coordinate election activity through a common vendor.

A Cambridge executive told FEC investigators on tape that the tech firm pushed positive messages online through the Trump campaign while negative material was pumped through outside groups such as Bannon's super PAC, which set up the "Crooked Hillary"-style attack ads.

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FEC investigators found that "plainly indicates" Cambridge was involved in the decision-making process for both committees, but the commission's Republican members may have helped shield Bannon and Trump from accountability in that probe.

"The FEC is a notoriously dysfunctional agency, and the GOP commissioners in particular have found every excuse they can to avoid enforcing the law," Fisher said. "Here, even they couldn't deny the evidence, but narrowed the inquiry to effectively kill the investigation before it could reach fruition."