This dichotomy between the haves and have-nots isn’t new, but the economic strain is now bleeding from the lowest earners to the middle class, creating an even starker divide that some economists say makes the economy more susceptible to a downturn. The richest 10% of households are fueling nearly half of total US spending, thanks to a stock market surge that has boosted wealth and in turn propelled economic output this year. Meanwhile, lower-income families are pulling back in the face of tight budgets, still-high living costs and a raft of corporate layoffs.