WASHINGTON — Bernie Sanders' presidential campaign took the extraordinary step of filing a lawsuit against his own party Friday, sending shockwaves through a relatively sleepy Democratic nomination contest with a claim that party officials had helped front-runner Hillary Clinton by unfairly restricting access to a crucial voter database.

The dispute erupted earlier Friday with revelations that four Sanders campaign aides exploited a software glitch and gained access to Clinton campaign data that was stored in a voter file maintained by the Democratic National Committee.

The Sanders campaign fired one of the aides involved, but then the DNC punished the campaign by preventing access to the voter file, which campaigns use to identify and target supporters.


Early Saturday, the Sanders campaign said that its access to the voter database would be restored that morning.

"We are extremely pleased that the DNC has reversed its outrageous decision to take Senator Sanders' data," said Jeff Weaver, Sanders' campaign manager.

The intraparty tensions spilled out into the open on the eve of the final Democratic debate of the year, set for Saturday night in Manchester, N.H. It marked the highest-profile rupture among Democrats during a campaign season that has largely been characterized by near-constant discord among Republicans.

Initially, the Clinton campaign accused the Sanders campaign of stealing its proprietary information.

"The Sanders campaign has misrepresented fundamentally what took place here," Clinton campaign manager Robby Mook said Friday night. "This was an egregious breach. Our data was stolen. . . . This breach is totally unacceptable and may have been a violation of the law."

Later, though, the Clinton campaign softened its tone.

"We hope that the court will resolve this matter tonight and the Sanders campaign has access to their voter files right away, with adequate protections of our proprietary information, which we understand could be completed in short order,'' the campaign said in a statement.


The DNC said the Sanders campaign improperly accessed Clinton campaign data. DNC chairwoman Debbie Wasserman Schultz, a US representative from Florida, was arriving in Manchester at about the time the Sanders campaign filed its 12-page lawsuit at a federal courthouse in Washington.

The suit claims that the financial damage exceeds $600,000 per day and that the political damage is "incapable of measurement." Weaver, speaking for the Sanders campaign, said at a press conference earlier Friday before the matter was resolved that the "Democratic National Committee is actively trying to undermine our campaign.''

"Our position is that [Sanders campaign officials] need to provide us with a full accounting of exactly what information was accessed, who accessed it, and that the information has been scrubbed from their computers," Wasserman Schultz said at the Manchester-Boston Regional airport as she arrived. "They've been told that if they provide that to us, we'll be able to consider restoring their access."

The voter file is vital to modern campaigns that use data to identify likely voters, discover what their leanings are, and figure out how best to use their time and resources to identify their supporters in the weeks before election day. Those get-out-the-vote efforts are crucial, especially in a primary when just a few thousand voters in either direction can mean the difference between a win or a loss.

The DNC maintains the master list. Individual campaigns rent access to file and add their own data to it — gleaned from campaign workers placing calls and knocking on doors — but they also sign a contract agreeing not to access rival candidates' input. There are technological safeguards to prevent this as well.


Without access to the file, the Sanders campaign would be at a significant disadvantage as it heads into the primary season and wouldn't be able to capitalize on the work its workers put into it.

In addition to being locked out of access to the national database, the Sanders campaign would no longer be able to access the information it has gathered for key states such as Iowa or New Hampshire.

The Sanders campaign had previously paid $100,000 for access to the New Hampshire file, but because it is blocked from the national list it, is also blocked from the New Hampshire list, according to Ray Buckley, the chairman of the New Hampshire Democratic Party.

Access to the New Hampshire voter file is key for candidates because it has voter histories and notes on voter behavior from previous presidential primaries, including those who backed Clinton or Barack Obama in the 2008 campaign. Such state-specific information can help campaigns work more efficiently, and it's not accessible through the Democratic National Committee database.

Weaver said irregularities with the voter file have dated at least back to October, when the campaign warned that data was not secure.

"We were assured that this was going to be taken care of," he said. "But apparently they are not competent in terms of maintaining the security of their data between the campaigns."


"We're running a clean campaign," he added. "We don't need dirty tricks."

The alleged data breach occurred on Wednesday when the vendor that maintains the database had a bug in a new code that was released. The vendor, NGP VAN, has a major office in Somerville's Davis Square, where most of its engineering work is done.

"Only one campaign took actions that could possibly have led to it retaining data to which it should not have had access," Stu Trevelyan, the company's chief executive, said in a statement. "This bug was a brief isolated issue, and we are not aware of any previous reports of such data being inappropriately available."

Wasserman Schultz said that "multiple staffers" from the Sanders campaign spent 45 minutes on Wednesday accessing and downloading data that had been collected by the Clinton campaign. She said the DNC has been asking since Wednesday for a full accounting of the Sanders campaign's actions, and so far has not received it.

"The bottom line is we need an independent audit, so that we can have an objective analysis of exactly what happened," she said in Manchester. "You know, there is no question that a window opened due to a software patch glitch, on the part of our vendor. But just because you have opportunity doesn't mean you should take advantage of it, clearly there are staff in their campaign that did."

She also said that while the vendor was at fault for allowing the breach, the Sanders campaign was at fault for exploiting it.


"Certainly the vendor made a mistake," she said. "But, people make mistakes leaving their front door unlocked also. It doesn't mean it's OK to go into their house and take their stuff, which is what their campaign staff essentially did."

As soon as the DNC learned about the data breach, Wasserman Schultz asked the vendor to shut off the Sanders campaign's access "until the DNC is provided with a full accounting of whether or not this information was used and the way in which it was disposed."

Josh Uretsky, the aide fired by the Sanders campaign, admitted on MSNBC that he had accessed the information but said he was trying to figure out the extent of the security problems before reporting it to the DNC.

The dispute comes nearly six weeks before the Iowa caucus kicks off the nominating contest, and at a time when Sanders is attempting to gain ground on Clinton, the front-runner.

A bitter back and forth could damage Democratic Party unity by infuriating the passionate supporters that Sanders has attracted to his campaign.

Sanders has also criticized the DNC for not holding enough debates, and for timing them during moments that seem designed to attracted a limited audience. Like the Saturday night debate, the face-offs so far have taken place on weekend evenings when many voters are not paying attention to politics.

James Pindell and Annie Linskey of the Globe staff contributed to this report. Matt Viser can be reached at matt.viser@globe.com.