David Brock and some of the biggest donors from across the left are rallying behind an effort to stave off a Republican avalanche in state-level races that could bury Democrats for decades.The effort, affiliated with a previously low-profile group called the State Innovation Exchange (or SiX for short), aims to offset Republican advantages in state-level campaigns, policy debates and looming redistricting fights.But SiX’s aggressive play could also set up a clash with other Democratic big-money efforts to boost the party at the state level, including an ambitious redistricting initiative backed by former President Barack Obama and his former attorney general, Eric Holder.The increased sense of urgency — and competition — among Democrats to reverse the party’s flagging fortunes in state houses provided the backdrop for the move by SiX, which was founded in 2014, to bring on a raft of big names of its own from across the Democratic spectrum.Brock, the self-described reformed right-wing hitman who became a key figure in Hillary Clinton’s unsuccessful presidential campaign, has been named co-chair of the beefed-up board of SiX’s political arm SiX-Action, along with former Texas state Sen.