Less than 12 hours after Bernie Sanders received his biggest endorsement of the presidential campaign, the Democratic National Committee (DNC) has cut off his access to the Democratic party’s all-important master voter file. The result is that his campaign is now unable to perform all the most basic aspects of voter contact such as knocking on doors and making phone calls in any functional way. While the Sanders campaign can still talk to voters, it has no way of actually keeping track of their identity and whether they support the Vermont senator’s candidacy.

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After NGP-VAN – the company that administers the DNC’s voter file – updated its system on Wednesday, a glitch reportedly made some confidential data from Hillary Clinton’s campaign briefly accessible to the Sanders campaign and one campaign staffer was able to access that information. The staffer has since been fired. The glitch was first reported by the Washington Post.

As a result of this temporary breach, the DNC has indefinitely cut off the Sanders campaign’s access to the voter file, which functionally halts its field operation. The move by the DNC raises eyebrows as many Democrats, including Sanders and fellow presidential candidate Martin O’Malley have long accused the DNC’s chair, Debbie Wasserman-Schultz, of attempting to rig the presidential process to benefit Clinton. In particular, they have raised questions about the relative paucity of debates, which have been scheduled for weekend evenings and to coincide with other events such as a major University of Iowa football game.

The action by the DNC happens less than 48 hours before the third Democratic debate, which is scheduled for Saturday night, only a few days before Christmas and at the same time as a nationally televised NFL game. The news also broke less than 12 hours after Sanders received his biggest labor endorsement yet from the Communication Workers of America as well as the backing of the influential progressive activist group Democracy for America. However, national polls show Clinton with a steady lead of well over 20 percentage points over Sanders and the Vermont senator has struggled to keep pace with Clinton in the key early caucus state of Iowa as well.

In a statement, Michael Briggs, a spokesman for the Sanders campaign told the Guardian the issue was the result of systemic failures by NGP-VAN. “Sadly, the vendor who runs the DNC’s voter file program continues to make serious errors,” said Briggs. “On more than one occasion, the vendor has dropped the firewall between the data of different Democratic campaigns. Our campaign months ago alerted the DNC to the fact that campaign data was being made available to other campaigns. At that time our campaign did not run to the media, relying instead on assurances from the vendor.”

He added of Wednesday’s incident: “Unfortunately, yesterday, the vendor once again dropped the firewall between the campaigns for some data. After discussion with the DNC it became clear that one of our staffers accessed some modeling data from another campaign. That behavior is unacceptable and that staffer was immediately fired. We are as interested as anyone in making sure that the software flaws are corrected since mistakes by the DNC’s vendor also have made our records vulnerable. We are working with the DNC and the vendor and hope that this kind of lapse will not occur again.”

Stu Trevelyan, the chief executive of NGP-VAN, told the Guardian: “The security and privacy of our customers’ data is our top priority. This was an isolated incident where as the result of a software patch, for a brief window, the voter data that is searchable across campaigns in VoteBuilder included specific data points it should not have, on a specific part of the system.”

Trevelyan, whose company is the dominant provider of software and data for Democratic political campaigns and US progressive causes, added: “We are conducting a full audit to ensure the integrity of the system and security of the data, and reporting on the findings to the DNC.”

A spokesman for the DNC didn’t respond to request for comment from the Guardian.