 -- A Bernie Sanders campaign staffer has been fired after allegedly accessing modeling data from the Clinton campaign, a spokesman for Sanders said, resulting in the Democratic National Committee suspending the Sanders campaign's access to its party voter data.

The move could seriously affect the Vermont senator’s field organizing and campaigning efforts just weeks before the first primary votes are cast.

"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," DNC spokesman Luis Miranda told ABC News in a statement. "The DNC immediately directed NGP VAN 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.

The statement continued: "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 said it was working with the 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."

Michael Briggs, spokesman for the Sanders campaign, blamed the incident on the software vendor used by the DNC.

“Sadly, the vendor who runs the DNC’s voter file program continues to make serious errors,” Briggs wrote in a statement to ABC News. “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.”

Conversations between the DNC and the campaign revealed that one of Sanders’s staffers had accessed Clinton campaign data, a move that the Sanders team called “unacceptable,” and the staffer was let go, Briggs said.

“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,” the statement from Briggs continued.

Briggs acknowledged that having access to the DNC’s data was important and said the campaign was working to resolve the issue. He added that he was “confident” the situation would be worked out.

A Democratic official told ABC News, "The Sanders campaign’s credentials for using the voter data system (basically, the login) have been suspended until the vendor and the DNC can review exactly what happened, and to make sure the Sanders campaign didn’t preserve a copy of any of the data it in appropriately accessed.”

"The voter file is a mammoth undertaking that all Democratic candidates have access to, the accumulated information about where voters live and how likely they are to vote and what party they’re likely to vote for. Not having access to it in the run-up to early-voting states would be very problematic for a field organization; the DNC doesn’t want to keep any of its candidates from the data, but wants to make sure everyone figures out what happened before letting the Sanders folks back in to the system.

"The DNC may order an independent audit by a data security firm, in addition to the review the company is running. So it might take some time to sort out."

The Clinton campaign had no comment.

The news comes at the end of what had been seen as a surprisingly good day for Sanders. He received two high-profile endorsements, one from the national Communications Workers of America union and another from the progressive activist organization Democracy for America. The campaign also trumpeted a milestone fundraising mark Thursday morning -- 2 million contributions, from over 800,000 donors, according to the campaign.

It all happened Wednesday when the vendor who maintains the voter file for the DNC applied a “software patch.” As a result of that, every campaign could have accessed the data all the others were – a glitch. The Sanders camp noticed it and accessed the Clinton files. Then they (and another campaign) told the company, and within half an hour they shut it down.