Prior to the 2010 election, the Alabama House had 60 Democratic members, 34 of them white and 26 black. Afterward, there were 36 Democrats—ten white, 26 black. Meanwhile, in the Alabama Senate, the number of black Democrats remained seven, while the number of white Democrats fell from 13 to four. The casualties included Barron, who lost to a first-time Republican candidate.
All of this was enough to give the GOP supermajorities in both chambers. Hubbard assumed his role as speaker of the House, and Marsh was elected Senate president pro tem. Having wrested control of the statehouse, now they could begin to change the state.
Clearly, for a long time, the Democratic Party ran things in the South by uniting who constituencies (white and black) who had little politically in common. As the parties have both increasingly nationalized in their agendas, whites started voting for the Republicans who more closely fit their politics while the black majority districts remained intact but are no longer able to achieve much since they are part of a minority party in a deeply divided legislature.
This nationalization of the political issues is apparent in some of the bits where it talks about Sanders' work in the legislature: fighting hard to block legislation which would restrict availability of abortion to children and prevent people from having to look for work before getting welfare. With priorities like this, it's not surprising that conservative southern whites don't vote for the Democrats anymore -- but the incidental casualty of the self marginalization of the Democrats is their inability to put through legislation to help their local communities.