Bernie Sanders's campaign just suffered a serious blow. Sanders can no longer access the Democratic National Committee's database of registered voters, after a glitch allowed the campaign to download data from rival Hillary Clinton.

This is a big setback, if the DNC doesn't restore Sanders's access quickly. The voter list is crucial for organizing voters in the run-up to the Iowa caucuses, where votes take place in less than six weeks.

The Sanders staffers were allowed access to Clinton's data in the first place due to a software error, the Washington Post reported. Four of them looked at Clinton's data, three at the direction of their boss, Josh Uretsky, who was fired.

The data was only available for about 30 minutes, according to Buzzfeed. The Sanders campaign said this type of error has happened with the database vendor, NGP VAN, before.

Uretsky told CNN he was just trying to assess the extent of the damage, and to see if the same glitch had affected the Sanders campaign: "To the best of my knowledge, nobody took anything that would have given the (Sanders) campaign any benefit."

Still, the DNC has cut off access until the Sanders campaign can fully explain itself and prove it's deleted Clinton's data.