NEW YORK (Reuters) - A decline of 10% in the benchmark S&P 500 stock index before the U.S. presidential election in November is "highly likely," Morgan Stanley Chief Investment Officer Mike Wilson said in an interview on Monday with Bloomberg TV.

Among the reasons for a decline are uncertainty over how swiftly the Federal Reserve will bring interest rates down from nearly two-decade highs and falling pricing power on the part of companies, increasing the likelihood of disappointing earnings results, he said.

"The average company has not had good earnings results," he said, adding that a nearly 17% gain in the S&P 500 for the year to date has been powered by a small number of companies.

At the same time, price to earnings multiples have been rising. "Valuations to me look very unexciting," he said.

Wilson maintained a bearish outlook for the majority of this year, one of few prominent forecasters to do so. In late May, he lifted his base-case 12-month forecast for the S&P500, an estimate of what the fair value of the index will be in a year, to 5,400 points. At the time, that was only 2% above its level but 20% higher than his previous forecast of 4,500.

The S&P 500 closed Monday at 5,572, some 3% above Wilson's 12-month target price.