The Democratic National Committee has told the campaign of US Senator Bernie Sanders of Vermont that it was suspending its access to its voter database after a software error enabled at least one of his staff members to review Hillary Clinton's private campaign data.
The decision by the party committee is a major blow to Sanders' campaign. The database includes information from voters across the nation and is used by campaigns to set strategy, especially in the early voting states.
The breach occurred after a software problem at the technology company NGP VAN, which gives campaigns access to the voter data. The problem inadvertently made proprietary voter data of Clinton's campaign visible to others, according to party committee officials.
The Sanders campaign said that it had fired a staff member who breached Clinton's data. But according to three people with direct knowledge of the breach, there were four user accounts associated with the Sanders campaign that ran searches while the security of Clinton's data was compromised.
The Washington Post first reported the issue.
The suspension comes as Sanders prepares to face Clinton at the Democratic debate Saturday and as he received the endorsement of a major union, the Communications Workers of America, and a liberal group Thursday.
The Democratic committee blamed NGP VAN for the software glitch.
"This was an isolated incident, and we're conducting a full audit to ensure the integrity of the system and reporting the findings to the DNC," Stu Trevelyan, NGP VAN's chief executive, said.
"The DNC was notified on Wednesday by its data systems vendor NGP VAN that as a result of a software patch, all users on the system across Democratic campaigns were inadvertently able to access some data belonging to other campaigns for a brief window," said the committee's communications director, Luis Miranda.
"The DNC immediately directed NGP VAN," he said, "to conduct a thorough analysis to identify any users who accessed the data, what actions they took in the system, and to report on the findings to the party and any affected campaign."
He added: "We have also instructed NGP VAN to conduct a full audit of the system to ensure the integrity of the data and the security of the system for the campaigns that use it, and to begin a review process with every campaign and user to ensure they understand and abide by the rules governing the use of the system.
"The DNC places a high priority on maintaining the security of our system and protecting the data on it," he continued. "We are working with our campaigns and the vendor to have full clarity on the extent of the breach, ensure that this isolated incident does not happen again, and to enable our campaigns to continue engaging voters on the issues that matter most to them and their families."
In a statement, Sanders' campaign spokesman, Michael Briggs, blamed the vendor for continuing to "make serious errors."
"On more than one occasion, the vendor has dropped the firewall between the data of different Democratic campaigns," he said. "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."
"Unfortunately, yesterday, the vendor once again dropped the firewall between the campaigns for some data," Briggs said. "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."
He added that the errors had also "made our records vulnerable." Despite repeated requests, the Sanders campaign did not provide information about the other user accounts that were involved.