A 20-ounce packet of Sara Lee Classic White Sandwich bread goes for $2.50 at Kroger, compared to $2.24 at Walmart.
Nearly two-thirds of Kroger's 2,700 stores are unionized, like a "majority" of Albertsons stores, the United Food and Commercial Workers Union says on its website.
Grocery stores such as Kroger, Albertsons, in contrast, are often forced to rely on coupons or buy-one-get-one-free promotions funded by companies like P&G and Conagra in order to compete.
Euromonitor data shows that 25.2% of all dollars spent on groceries in the United States last year went to Walmart, while Kroger got 8.1% and Albertson's 4.8%.
Walmart shoppers have a median income of $73,000.