The U.S. stock market shrugged off all manner of political uncertainty in 2017, essentially ignoring heightened tensions with North Korea, populist movements across the globe, and a number of terrorist attacks and mass shootings as major indexes marched to dozens of records with basically no volatility or pullbacks.

In 2018, investors might not be so lucky.

According to Joshua Brown, CEO of Ritholtz Wealth Management and a closely watched market commentator, the “lackadaisically bullish consensus” for equity gains in 2018—driven by synchronized global growth and improving corporate profits, among other factors—is failing to account for what he called “one major shock” that could potentially hit stocks.

Read:Global stocks could make history this month by refusing to fall

The shock is connected to President Donald Trump and the investigation currently being conducted by special counsel Robert Mueller into Russia’s interference in the 2016 election and other issues arising from that. Trump has repeated denied the allegation that anyone associated with his campaign colluded with Russia, calling the probe a “witch hunt.”

There are indications the investigation is nearing Trump’s inner circle. Earlier this month, former national-security adviser Michael Flynn pleaded guilty to lying to the Federal Bureau of Investigation and promised full cooperation with the special counsel’s investigation. Separately, Deutsche Bank AG has reportedly been told to hand over information about its dealings with Trump, crossing what the president had previously deemed a “red line” he wouldn’t allow. (President Trump’s lawyer Jay Sekulow and White House press secretary Sarah Sanders both stated that Deutsche Bank hadn’t been subpoenaed.)

Opinion:Stock bulls, watch out because Trump’s ‘red line’ may have been crossed

“ ‘My personal belief is that this possibility is not being priced into risk assets. And lest you think this is an event that the rest of the country will accept tacitly, sending off a few tweets and then going to bed, you may want to rethink this.’ ” — —Joshua Brown, CEO of Ritholtz Wealth Management

Brown speculated that the investigation could reach people ever closer to Trump, if not the president himself. (Paul Manafort, Trump’s former campaign manager, was also charged as part of the investigation, while a former campaign aide pleaded guilty to lying to federal agents.) There has also been speculation that Trump, in an attempt to halt the investigation, would fire Mueller, something Brown views as a near certainty.

“Now there is talk in the air of a Constitutional Crisis when (not if) Mueller gets too close to the truth and gets removed from the investigation,” Brown wrote in a blog post. “My personal belief is that this possibility is not being priced into risk assets. And lest you think this is an event that the rest of the country will accept tacitly, sending off a few tweets and then going to bed, you may want to rethink this.”

Read:Here’s how Trump could fire Robert Mueller, the special counsel

According to The Wall Street Journal, Trump’s legal team has mostly cooperated with the investigation, although the president is being urged to adopt a more hard-line approach. Axios reported that “Trump’s lawyers say they respect Mueller and trust him, and want to get to the finish line with him.” The White House didn’t immediately return a request for a comment.

Thus far, the S&P 500 SPX, -2.37% has only shown slight correlation with the Mueller investigation. Earlier this month, major indexes plummeted in midday trading after the Flynn news was first reported. The volatility proved short lived as investors turned their focus back to the prospects for tax-reform legislation and economic fundamentals, but it was a signal of how traders are closely watching this story, and how the record levels of stocks could be vulnerable in the event there are more developments.

The tax bill “swept those [Mueller-related] fears under the rug a few days later, but it didn’t stamp them out completely,” Brown wrote. “They’re still with us, floating around in the ether, stored up as potential energy in the lines of algorithmic code, just waiting to be unleashed by machines and people at a moment’s notice.”

While a spike in political uncertainty could bring volatility back into markets, particularly since the Cboe Volatility Index VIX, +6.40% , one measure of implied volatility in the S&P 500 index is trading at historically low levels, the long-term impact is difficult to determine. If the investigation limits Trump’s ability to pass his legislation through Congress, something the market has been rising in anticipation of, that could limit the upside potential on Wall Street. However, the investigation wouldn’t have a direct impact on the pace of economic growth or the labor market, two factors that have also been supporting equity prices.

Brown speculated that domestic unrest could spike if Trump were to remove Mueller, especially because he doesn’t expect the Republican-led House or Senate to explore impeachment under any scenario.

”Because Congress will not act to defend the Constitution, the people are going flip out when it goes down. The somewhat amorphous #Resistance movement is going to explode into the streets, making last fall’s Women’s March look like a May Day Parade,” he wrote.

There is limited historical precedence for unrest leading to prolonged market turbulence. Protests or even riots tend to have a limited economic effect, especially for multinational companies with global revenue exposure, and particularly as more Americans work from home and shop online. Wall Street shrugged off violence in Charlottesville, Va., earlier this year, and rose throughout 1964, a turbulent year marked by demonstrations and violence.

Read more:Here’s why domestic unrest isn’t a worry for Wall Street

Brown didn’t provide investment advice for how to position in the event political uncertainty returns, writing, “I don’t think it’s mandatory that an investor has a reaction in mind for when this takes place.”

He added, however, “Steeling ourselves mentally in advance of the event may be an even better solution given the speed with which these events are digested these days. In either case, I think the Mueller firing ought to be front of mind for all of us right now. It’s precisely the sort of exogenous shock that no one seems to be counting on.”