The Democratic presidential frontrunner offered what appears to be an unvarnished mea culpa over her decision to use a private email server while Secretary of State in an interview that will air later tonight on ABC News. “That was a mistake. I’m sorry about that. I take responsibility,” Clinton told the network’s David Muir in a New York City sit-down on Tuesday, according to a snippet released by ABC.

The candidate’s offer of contrition comes a day after she told the Associated Press she would not apologize because her email setup conformed with federal rules.

The issue has cost Clinton in polls, both nationally and in the early states of Iowa and New Hampshire, where she has seen once-commanding leads over her nearest challenger, independent Vermont Sen. Bernie Sanders, all but collapse. And questions about whether Clinton improperly transmitted classified material from her home-brew server has touched off a federal security review involving the Justice Department.

ABC will broadcast the full interview tonight at 6:30pm Eastern and again at 12:30am.