Stocks tumbled hard and in heavy trade at the open Tuesday. The Dow and the S&P 500 shed 2% apiece. The Nasdaq was down 1.7%. Retailers, oil producers and consumer electronics makers were among the worst early losses. Volume swelled — an unusual event in the week ahead of the Labor Day holiday. Trade rose 51% on the Nasdaq and was 40% higher on the NYSE in relation to action at the same time Monday.