Massachusetts Sen. Elizabeth Warren signed a secret letter from U.S. Senate Democratic women to Hillary Clinton encouraging her to run for president in 2016, a signal that the liberal favorite is not positioning herself for an insurgent candidacy of her own.

ABC News confirmed Warren signed onto the letter that was drafted by California Sen. Barbara Boxer and meant to be a private show of support for Clinton by 16 female senators.

But North Carolina Sen. Kay Hagan let the cat out of the bag unintentionally, referencing the letter during an EMILY's List event Monday.

The RUN contacted both Hagan and Warren's offices Tuesday night and has not heard back from either.

ABC reports the slip-up prompted Hagan to apologize to her colleagues.

Warren is increasingly cited by the Washington punditry class as an anti-establishment force who could command the stature and resources to mount a challenge to Clinton in a Democratic primary.

Her signature on a letter prodding Clinton into the presidential race makes that prospect a bit less likely.

