WASHINGTON — The Democratic National Committee’s highest-ranking aide abruptly quit Monday, the latest setback for a group that has struggled with fund-raising and organization even as the party’s candidates have found recent success.

Jess O’Connell, the committee’s chief executive, announced her departure in an email message to other party staff members, declaring that she was “proud to have helped rebuild” the committee but not offering any reason for her resignation.

But senior Democratic officials indicated that Ms. O’Connell, a former top official at Emily’s List, clashed over authority with Thomas E. Perez, the committee’s chairman, who narrowly won his post last year in an election that revealed the lingering fissures between the party’s establishment and progressive wings.