The economic headwinds and downbeat sentiment have forced investors to consider to what extent those recent data points could spell more pain ahead for U.S. equities. Despite the recent market turmoil, the major U.S. indexes are still outperforming their global peers. The S&P 500 is off more than 12 percent from its late September record, while the Dow Jones Industrial Average has dropped more than 11 percent from its peak in early October. By comparison, the pan-European Stoxx 600 index has fallen more than 15 percent from its 52-week high.
But recent signs have suggested the slowing growth could be catching up with U.S. markets. On Monday, the Dow finished at its lowest levels since late March and the S&P 500 posted its lowest close since October 2017. Weak economic data from China has drowned out hopes of progress in ongoing trade talks. The Dow, S&P 500 and Nasdaq Composite are all sitting in correction territory for the first time since March 2016.
"We've been cautious for quite a while ... and we're assuming that we're in the midst of a new cyclical bear market," Doug Ramsey, chief investment officer at The Leuthold Group, told CNBC. "If you look at declines in foreign stocks, you don't even have to make that forecast. It's there, and it's happened. The Dow and the S&P 500 are always the ones that put on this show of superficial strength right up until the very end, and I think they'll be the last to crack into next year."
Weakness in key sectors has also weighed on investor confidence. Financials fell into a bear market on Friday, joining materials and energy stocks. The decline was driven by a fall in bank shares amid worries about slowing global economic growth.
And other economically sensitive sectors, including housing, transport and industrial stocks, are all down by double digits in 2018.
DoubleLine Capital CEO Jeffrey Gundlach sees more pain ahead, saying that he "absolutely" believes the S&P 500 will go below the lows that the index hit early in 2018 amid these economic warning signs.
"I'm pretty sure this is a bear market," Gundlach told Scott Wapner on CNBC's "Halftime Report" on Monday. "We've had pretty much all of the variables which characterize a bear market."
Gundlach specifically noted that 80 percent of the iShares MSCI World Index — which tracks global markets across the world including the U.S. — is currently in a "death cross". Wall Street traditionally defines a "death cross" when a stock or index's average price over the last 50 days drops below the 200-day moving average, a sign of negative momentum and possible change in trend.
"It's a pretty widespread and coordinated set of weaknesses," Gundlach added.
Several investors agree that U.S. equities will continue to be vulnerable heading into next year, but not all are convinced major U.S. indexes will follow in lockstep with these other world markets. Jeffrey Saut, chief investment strategist at Raymond James, and Anastasia Amoroso, head of investment strategy at J.P. Morgan Private Bank, both pointed to high projections for corporate earnings growth next year as an encouraging sign.
Others are less convinced of a steep U.S. economic contraction.
"The fact that Germany is down 20 percent, China is down 20 percent, and now the U.S. has joined the downturn ... that's what suggests to us that the global economy is in recession or on the verge of recession," Bruce Bittles, Baird & Co. chief investment strategist, told CNBC last week. "[But] the U.S., we don't think, is going to enter recession. We think there's going to be a slowdown here ... that's going to influence corporate profits to a certain extent."
Bittles added that volatility would likely remain high going into 2019, and "persistent downside momentum" remained a key obstacle to a turnaround.