America’s millionaires are betting on a strong stock market next year, with many expecting double-digit gains, according to the CNBC Millionaire Survey.Fully 70% of millionaires expect the S&P 500 will be up at least 5% next year, according to the survey of households with investible assets of at least $1 million. Nearly a third of those polled expect gains of at least 10% in 2021, according to the survey.The S&P 500 is heading into the final weeks of 2020 up nearly 14%. Despite market levels being near or at record highs, most millionaires plan to maintain or add to their stock holdings next year, according to the survey. Only 9% plan to decrease their ownership of stocks. That bullishness in the face of Covid pandemic and economic uncertainty could be a strong tailwind for the market given that millionaires own more than 85% of individually held stocks.”I expected to see more of them making plans to get out of the market at these levels,” said George Walper, president of Spectrem Group, a research and consulting firm. “They have clearly adjusted to the volatility.”Half of millionaires surveyed expect their personal wealth will increase next year, while another 40% say they expect it will stay the same. Millionaire views on the biggest financial risks next year depend more on political party affiliations than wealth levels. Republican millionaires cited taxes as the biggest risk to their wealth in 2021, while Democratic millionaires cited the virus as the biggest risk.Less than a third of millionaires say the economy will be weaker next year, and most say interest rates will stay the same, which will also help stocks. Their bright outlook is yet another sign of a K-shaped recovery where middle and lower earners and small businesses are still struggling while upper-income workers and investors continue to see improvements.Equities are the favorite investment category for millionaires next year, with 44% planning to add stocks. When asked which market sectors they plan to add more money to in 2021, technology ranked first, followed by health care and financials.”Given all that’s going on around us, they are optimistic,” Walper said. “It’s the vaccine and the hope for a reopening.”Spectrem Group conducted the survey for CNBC. It polled 750 participants with investible assets of $1 million or more in November.