if the us had always been a parliamentary system or became one after the civil war or something i can imagine things like universal healthcare being easier to pass during or after the great depression and the red scare might not have been as bad but if we woke up tomorrow and magically had a parliament i don't think it would change too much, it seems like in this age of neoliberalism even if third parties wind up winning seats or even a majority they simply become a slightly different flavor of libs