Investors and businesses may have overlooked one big risk that could come out of Washington next year: The possible impeachment of U.S. President Donald Trump.

Markets are bracing for a more difficult 2019, with risks such as interest rate hikes from the Federal Reserve and an escalation in U.S.-China trade tensions at the top of many people's minds. But the possible impeachment of Trump has rarely been discussed, partly because it still isn't clear whether Democrats will go down that route when they take control of the House in 2019.

Being impeached doesn't mean being kicked out of office. On the U.S. federal level, it's a process whereby a simple majority vote in the House can accuse a civil official, including the president, of an "impeachable" offense. That accusation would then move to the Senate, which would decide whether to convict the official.

For more on the details of impeachment, see this explanation.

Whether or not the impeachment process gets underway, it's likely to feature highly in U.S. political discussions and cripple Washington's ability to get things done — that in itself is a big risk for businesses, said Steve Okun, senior advisor at U.S.-based geopolitical consulting firm McLarty Associates.

"Democrats will do something on impeachment next year," he told CNBC's Nancy Hungerford on Dec. 26.