Spectrem's Millionaire Investor Confidence Index hit a four-month low in September, propelled by the uncertain political landscape.

Millionaire and affluent investor confidence dropped in the US last month, primarily due to the current political climate, according to Spectrem Group.

The firm’s most recent High Net Worth Insights newsletter says millionaire investor confidence fell 7 points to 5 in September, while confidence among affluent investors dipped 3 points to 4.

This is the lowest total for the millionaire index since 2012, when the index stood at 1 in the run-up to that year's Presidential election, Spectrem noted. It added that the one point separating the millionaire and the affluent indices last month is the smallest gap since January.

“While millionaires generally have a more optimistic investment mindset than non-millionaires, the narrowing of the gap indicates a growing level of wariness among more affluent investors,” Spectrem said.

Beyond the political environment, other factors contributing to a more negative investor mindset in September included a recent lackluster US jobs report and the potential for a US interest rate hike before year-end, according to the firm’s president, George Walper.

The indices were launched in February 2004 by Spectrem to measure the investment confidence and outlook of the 16 million US households with over $500,000 of investable assets, and those with $1 million or more.

They read as: -31 to -51 is bearish; -11 to -30 is mildly bearish; 10 to -10 is neutral; 11 to 30 is mildly bullish; and 31 to 51 is bullish.