is described by me over at Daily Kos. The argument hinges on the notion that what works to help a party in 3-seated state assembly elections is what hurts them in 1-seated state senate elections. As such, they'd be in a Catch-22 situation if they tried to game the state legislative districts. And, the 3rd party wild cards wd disrupt voter patterns by encouraging more habitual nonvoters to vote strategically in major elections. This would also subvert the ability to gerrymander US congressional elections!
I also describe briefly in the diary linked to above how if we trusted our state assemblies more that we could give them back their constitutional right to elect our US senators and come out ahead in many important respects!