The holiday season is expected to be difficult for most retailers, but it could prove especially tough for one of the nation’s most storied brands: Sears. Sears, a fixture of American retailing for more than 100 years, had already been struggling to find its niche before the economic downturn began in earnest.Now, it’s facing the double whammy of a dismal housing market, which is crimping sales of flagship items like appliances, and cash-strapped holiday shoppers looking for the biggest bargains on other discretionary items, like clothes.