Democratic presidential candidate Bernie Sanders' campaign has had its access to a critical voter database suspended following a data breach.
The campaign has fired a member of staff who had viewed data owned by the Hillary Clinton campaign in the party-controlled database.
The Sanders campaign and the Democratic party have blamed the software vendor.
The party has asked for a full investigation into the software.
Robby Mook, the campaign manager for the Clinton campaign, said that "this was a very egregious breach and our data was stolen".
Sanders campaign officials blamed a software glitch for allowing the low-level member of staff access.
However Congresswoman Debbie Wasserman Schultz, the chairwoman of the Democratic National Committee (DNC), said multiple Sanders staff members accessed Clinton campaign data and downloaded it.
The Sanders campaign on Friday filed a lawsuit against the DNC in a federal court to restore its access to the voter data.
Sanders campaign manager Jeff Weaver called the DNC's actions "a heavy-handed attempt to undermine our campaign".
Strategically important information on voters is contained in the database, which campaigns use to decide strategy.
That data takes on a crucial role as campaigns prepare for early primary voting in just over a month's time.
Forbidding the Sanders campaign from accessing the database will significantly hamper its ability to reach out to new Democratic voters, says the BBC's Anthony Zurcher in Washington.
The DNC maintains the database and charges campaigns for access.
The campaigns then add their own data gathered by volunteers and field workers; this data, when the software is working correctly, cannot be accessed by other campaigns.
During a software update on Wednesday, members of staff at the Sanders campaign were inadvertently granted access to the private data stores of the Clinton campaign.
The DNC has suspended the Sanders campaign's access to the database until it provides an explanation of why the data was accessed and guarantees that any and all data proprietary to the Clinton campaign that it may have a copy of is destroyed.
"After discussion with the DNC, it became clear that one of our staffers accessed some modelling data from another campaign," Sanders campaign spokesman Michael Briggs said in a statement quoted by the New York Times.
"That behaviour is unacceptable and that staffer was immediately fired."
Primary voting begins in early February.