WASHINGTON — Bernie Sanders’ presidential campaign filed a lawsuit against the Democratic Party on Friday after it was temporarily barred from accessing a trove of information about potential voters as a punishment for improperly accessing data compiled by the campaign of rival Hillary Clinton.

The reaction of the Democratic National Committee to the data breach, the depth of which was debated by all involved, thrust into the open long-standing suspicions among Sanders and his supporters that the national party is unfairly working to support the candidacy of its front-runner.

“Clearly, in this case, they are trying to help the Clinton campaign,” said Sanders campaign manager Jeff Weaver.

DNC Chairwoman Debbie Wasserman Schultz responded that “the Sanders campaign had inappropriately and systematically accessed Clinton campaign data,” rejecting Weaver’s effort to portray the breach as the fault of a software glitch and a small group of rogue staffers.

The lawsuit, filed in U.S. District Court in Washington, sought the “immediate restoration” the Sanders campaign’s access to the voter database. It argued that without the database, the campaign would lose approximately $600,000 in donations a day.

“The loss of DNC support could significantly disadvantage, if not cripple, a Democratic candidate’s campaign for public office,” the lawsuit said.

The back-and-forth on the eve of the party’s next presidential debate underscored Sanders’ attempt to cast himself as an anti-establishment upstart willing to take on Clinton, the unquestioned front-runner for the party’s nomination.

But by firing his top data staffer and admitting that members of his staff looked at information that belonged to the Clinton campaign, Sanders also threatened to undercut his image as an honest broker seeking to foster a “political revolution” to help the nation’s poor and beleaguered middle class.

The incident interrupted a period in which Democrats were sailing toward a peaceful primary season, with Clinton comfortably ahead of Sanders nationally in a campaign that harbors little of the discord and discontent roiling the Republican Party.

Sanders’ campaign quickly sought to make the most of the dispute, saying in a fundraising email that its “quick rise in the national polls (has) caused the Democratic National Committee to place its thumb on the scales in support of Hillary Clinton’s campaign.”

Notably, that email made no mention of the campaign’s decision to dismiss a staffer and Weaver’s admission that the staffer’s actions were “unacceptable.”

The DNC maintains an extensive database of voter information, which it rents to campaigns. The campaigns then update that database with their own information about voters. The data is used to target likely voters and anticipate what issues might motivate them to support a candidate.

The information is particularly important in the first states to vote in the presidential nominating process, when a campaign’s ability to organize its supporters and make sure they cast ballots can make the difference between winning and losing.

Firewalls are put in place to prevent campaigns from looking at data maintained by their rivals. But officials said the vendor that runs the system, NGP VAN, ran a software patch Wednesday that allowed all users to access data belonging to other campaigns.

“We were informed that our proprietary data was breached by Sanders campaign staff in 25 searches by four different accounts and that this data was saved into the Sanders campaign account,” said Clinton campaign spokesman Brian Fallon.

The breach did not involve any hacking or enable any voting information to enter the public domain, officials said.

The Sanders campaign accused NGP VAN of making “serious errors.” Weaver said four members of the Sanders campaign had accessed the information, but only the actions of one, the campaign’s data director, had risen to the level of a fireable offense.

Weaver argued the firewall used by the vendor had previously failed, and he railed against the party for not taking the steps required to keep the information secure.

“While that information was made available to our campaign because of the incompetence of the vendor, it should not have been looked at,” Weaver said.

Josh Uretsky, the data director fired from Sanders’ campaign, said his team was merely investigating the security problem and trying to figure out how exposed the software patch left their own data.

“I believe that I took appropriate steps to audit and assess the security breach and that nothing I did was done in a way that it would give the Sanders campaign a competitive advantage,” he said in an email to The Associated Press.

While Uretsky has taken responsibility for the incident, he didn’t believe the DNC would think he violated any rules. “We deliberately conducted that investigation in a way that we knew was tracked to make it clear that our intention was to be above board and transparent,” he said.

Summaries of data logs provided to AP show the Sanders team spent about an hour in the database reviewing information on Clinton’s high-priority voters and other data from nearly a dozen states, including first-to-vote Iowa, New Hampshire and South Carolina.

Some of these voter lists were saved into a folder named “Targets,” according to the logs. Uretsky’s deputy appeared to focus on pulling data on South Carolina and Iowa voters based on turnout and support – or lack of support – for Clinton.

Wasserman Schultz said the DNC had asked the Sanders campaign for a “full accounting of whether or not this information was used and the way in which it was disposed.” Only then will the party make a decision on restoring Sanders’ access to the database, she said.

That decision infuriated Weaver, who said the party had cut Sanders and his team off from the “lifeblood of any campaign.”

“This is information that we have worked hard to obtain,” he said. “It is our information, not the DNC’s.”

News of the data breach was first reported by The Washington Post.