After weeks of false starts, the Standard & Poor's 500-stock index finally broke a six-year-old closing high to mark a new record.

The push to record territory came in midmorning trading, as the S&P 500 broke past its previous record close of 1565.15 on Oct. 9, 2007.

The S&P 500 gained 6.34 points, or 0.4%, to 1569. The Dow Jones...