Their own (National Democratic Training Committee) polling
From the article:
So who are those candidates, exactly? They include Hillary Clinton and Joe P. Kennedy III. While Kamala Harris is first on the list, Clinton appears directly between Joe Biden and Bernie Sanders. This is no accident — the National Democratic Training Committee is also running a separate poll, the intro to which states that Clinton was “robbed” in 2016 and confirms, “we’re polling Top Democrats from your area to determine possible support for a Hillary Clinton 2020 Presidential Ticket.”
Wow.