Don’t look now, but Vermont Sen. Bernie Sanders is starting to turn heads and cause worry on Wall Street.

The self-proclaimed Democratic Socialist has gained a lot of ground on front-runner and former Vice President Joe Biden in Democratic primary polling, which has not gone unnoticed by investors. Shares of some of the country’s top health insurance companies are sinking this week upon news of polling data showing Sanders has actually passed Biden in Iowa, where the state’s Democratic caucuses are set to take place Monday.

Anthem (NYSE: ANTM), UnitedHealth (NYSE: UNH) and Cigna (NYSE: CI) have all seen their shares fall with the rise of Sanders, who is running on a Medicare for All platform that would wipe out private health insurance.

Friday afternoon, UnitedHealth was just under $300 a share and by this morning, it has fallen all the way under $283, a dip of nearly 6%. Anthem and Cigna have mirrored UnitedHealth’s fall over the same time period. The past week, Anthem is down 10%, UnitedHealth nearly 6% and Cigna more than 4%, all over fears that Medicare for All could happen.

“If we are going to break the stranglehold of corporate interests over the health care needs of the American people, we have got to confront a Washington culture that has let this go on for far too long,” Sanders said at an event over the summer. “That is why I am calling on every Democratic candidate in this election to join us in rejecting money from the insurance and drug industries.”

Medicare for All also was a cornerstone of Sanders’ losing campaign against Hillary Clinton in 2016.

The socialist senator’s rise also has coincided with the fall of rival Sen. Elizabeth Warren, who had shot to the top of the heap briefly in October.

“There was a battle between Sanders and Warren on who would be the progressive candidate, and Sanders appears to have won that battle,” Fundstrat Global Advisors’ Thomas Block told CNBC. “People I’ve talked to continue to not take Bernie that seriously.

“A big win in Iowa could change that.”

“Bond King” Jeffrey Gundlach, who predicted Donald Trump would win in 2016, has been telling anyone who will listen of late that Sanders is the biggest threat to the market this year as he is becoming “more believed in as a real force,” and people should absolutely take him seriously.

“Bernie is stronger than people think,” the DoubleLine Capital CEO said. “I think it will be taken more seriously as the field winnows. The financial markets broadly will have to deal with the fact that there could be a scare that Bernie Sanders is starting to become a plausible candidate for the nomination.”

Gundlach even believes Sanders could threaten incumbent President Trump in a head-to-head matchup if the economy starts to sputter.

“He would really need to push him into a majority position, you would need people to be soured even further on capitalism and more fond of socialism as a broad concept,” Gundlach said.

PredictIt has Sanders’ odds of clinching the Democratic nomination at 41% this morning, which is 8% ahead of second-place Biden.

FiveThirtyEight has Biden at a 45% chance to win the nomination, with Sanders trailing at 29%. However, Sanders has jumped to 29% from 20% earlier this month.

In addition to Gundlach, hedge fund titan Stanley Druckenmiller also said last summer that stocks would plummet if a self-described socialist enters the White House.

“If Bernie Sanders became president, I think stock prices should be 30% to 40% lower than they are now. The good news is we’d all be much more equal because everybody would be poorer but the rich would have lost a lot more wealth than the poor would have,” Druckenmiller quipped.