The Dow Jones Industrial Average was trading sharply higher midday Tuesday, putting blue chips on track to book the best one-day gain since March. The Dow was up about 492 points, or 2%, at 25,739. If it holds the gain would represent the best point and percentage climb since March 26. Tuesday's rally was partly on the back of a surge in shares of component UnitedHealth Group Inc., which was delivering a roughly 70-point jolt to the price-weighted equity gauge. Share rises for UnitedHealth come after the health-care company raised its full-year earnings outlook and said it continues to see growth in health-care plan membership and premiums. Meanwhile, the S&P 500 index was climbing 2% at 2,804 and the Nasdaq Composite Index advanced by 2.6% to 7,620. Wall Street investors appear to be focusing upbeat earnings, including those from Goldman Sachs Group Inc. , Johnson & Johnson and Morgan Stanley .