I don't know if it is, but his support base is clearly among those who want to 'take back' the Democratic Party from the Clintonistas. This is a silly and unachievable goal, as the Democratic Party has never been the party of the left-liberals and the labor movement, which have been the battered spouse of the party since they entered into an unholy alliance with finance capital in the 1930s and 1940s in opposition to the dominant Republican Party.
Really, the idea that the Democratic Party can be won over at all to left-liberal positions is what is entirely laughable, given the only time it did succeed to some extent (1972), the party quickly collapsed, losing most of its major donors and political support while also being painted as out of touch and unhinged by the media. You should expect the exact same to happen if Sanders or another left-liberal captures the nomination of the Democratic Party in the future.
Just a note. Organized labor, embodied in the AFL, has consistently supported the Democratic nominee since 1908. According to Jack Ross (
The Socialist Party of America: A Complete History, 107)
On the crisis within the AFL (Ross, 103):