Provided by Dow Jones By Michael Wursthorn Bruce Burnworth used to clip coupons and look for deals before his investment in Tesla Inc. made him a millionaire. He is part of a widening class of affluent Americans who are doubling, or even tripling, down on this year's highflying stock market. The S&P 500 has soared 66% since bottoming in March in the early days of the Covid-19 pandemic, while dozens of individual stocks, like Tesla, have surged even higher. Some investors have been tempted to chase bigger gains -- and have exposed themselves to potentially devastating losses -- through riskier plays, such as concentrated positions, trading options and leveraged exchange-traded funds. Others are borrowing against their investment portfolios, pushing margin balances to the first record in more than two years, to buy even more stock.