The Democratic National Committee has temporarily cut off Sen. Bernard Sanders‘ presidential campaign from accessing its voter files after a staffer viewed private voter information compiled by the campaign of 2016 Democratic rival and former Secretary of State Hillary Clinton.

Mr. Sanders‘ campaign has fired one staffer over the incident. His campaign manager blamed the software vendor hired by the DNC for a glitch that enabled the access, according to The Washington Post, which first reported the news.

“The DNC places a high priority on maintaining the security of our system and protecting the data on it,” DNC spokesman Luis Miranda said in a statement to The Post. “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.”

The news come as Mr. Sanders, Mrs. Clinton, and former Maryland Gov. Martin O’Malley prepare for the next Democratic presidential debate, to be held Saturday in New Hampshire.

It deals Mr. Sanders, who is trailing Mrs. Clinton in much of the polling on the race, a setback on the heels of two big endorsements for the Vermont senator from the Communications Workers of America, a union group, and the liberal advocacy group Democracy for America.

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