Hillary Clinton finally apologizes for private email server: 'That was a mistake. I'm sorry about that.'

Hillary Clinton speaks at Columbia University.

(Image credit: Kevin Hagen/Getty Images)

After months of defending her use of a private email server during her time as secretary of state, it seems Democratic presidential frontrunner Hillary Clinton has had a change of heart.

In an interview that will air Tuesday evening, Clinton sits down with ABC's David Muir, marking her second national interview in a matter of days, but only her fourth since her campaign launch. Ahead of the broadcast, Muir tweeted:

For months, Clinton has refused to apologize, claiming that what she did was allowed. Last year, Clinton released 55,000 pages of emails to the State Department, many of which have been released to the public since. Clinton has insisted that she "did not send or receive any information marked classified," though The New York Times reported this week that two emails received at her private email account were marked "Top Secret" at the time.