A staff member for Democratic presidential candidate Tom Steyer used access he had previously been granted while working for the South Carolina Democratic Party to download data about rival Kamala Harris' campaign, state party officials said Monday.

Democratic presidential candidate, philanthropist Tom Steyer speaks at the Liberty and Justice Celebration at the Wells Fargo Arena in Des Moines, Iowa on Nov. 1, 2019. Scott Olson / Getty Images

The South Carolina Post & Courier was the first to report the breach.

The staffer, Dwane Sims, resigned Monday after an internal investigation, according to statement from the Steyer campaign Monday night.

A South Carolina Democratic Party official said that Sims, who had served as the Steyer team deputy state director, was able to download volunteer data from the Harris campaign because he had created and retained access to a separate user account via his personal email.

The Steyer campaign on Monday said the Harris campaign information was inadvertently downloaded because of a glitch that happened when the state party restricted the Steyer campaign's access.

The temporary block was because of a miscommunication regarding payment, a senior campaign official told NBC News. Once that was resolved, Sims' access to the voter file was restored at the level that he had when he worked for the party, which is more expansive than what he should have had as a campaign staff member. Sims started downloading Harris' data, which he believed to be Steyer's data, according to the campaign official.

"Within minutes of realizing this, Mr. Sims called the South Carolina Democratic Party to let them know of their oversight," Benjamin Gerdes, Steyer's national press secretary, told NBC News. "He acted quickly to alert the appropriate people to rectify the matter, and the access was turned off by the party authorities."

The state party worked with the DNC to disable Sims' access and sent him a cease-and-desist letter, demanding that he delete all the downloaded information.

The Steyer campaign said that Sims had deleted all volunteer data from his computer and informed the DNC that he had done so. Sims' computer was also wiped by the campaign’s internal IT department as a precaution, according to the Steyer team.

"All data downloaded by this individual was destroyed and was not provided to any third parties. It is critical that the Steyer campaign take immediate action regarding their employee," SCDP Charmain Trav Robertson said in a statement. "This user account did not have access to data from any other presidential campaign."

The Harris campaign did not comment, but communications director Lily Adams tweeted Monday: "Can't buy hard work."