Cargo aircrafts from UPS, FedEx and West Air Europe are parked at Cargo City at Arlanda airport in Stockholm.

Transportation stocks have hit the skids and are looking worse than the broader market, a trend that needs to reverse for stocks to stabilize.

The Dow Transports have lost 15 percent since Oct. 1, 6 percentage points worse than the S&P 500, and they show no sign of improving. The Dow Transports are down nearly 9 percent year to date, while the Dow and S&P are both down less than 1 percent.

Transportation companies are the lifeblood of the economy, but shares of firms such as FedEx have been hit hard. The package delivery company is off more than 18 percent this month alone, and its rival UPS is down 12.6 percent in the same time frame. United Continental is down about 9.5 percent this month, and CSX is down 5.2 percent. Union Pacific, down 1 percent Thursday, was also down 5.2 percent for the month and 10 percent since Oct. 1.

The S&P is down 3.9 percent since the start of December, and the Dow is down 3.7 percent, compared with the 10.6 percent decline in the Dow Transports.

"We look at them as a leadership group, and that's the opposite of what's going on," said Todd Sohn, technical analyst with Strategas Research. "Oil going from $75 to $50 has done nothing for transportation stocks."

Sohn said along with the industrials, he watches the transports because they are important pillars for the stock market, as are financials and tech.

Source: Strategas

"I think they have a longer-range problem," said Art Cashin, director of floor operations at UBS. He said the group's troubles began with the announcement of tariffs. "It pulled forward a lot of business. People were importing a lot of stuff. They ordered a lot of stuff, and I think we're now seeing a slowdown in terms of activity."

Sohn said some of the group's better performers are now rolling over, like the railroads.

"There's a lot of names that have heavy technical damage. Some of them could rally over the next few weeks, like a FedEx, but I think the broader damage has been done. It's going to take some time to repair," Sohn said. "These were stocks that went up sharply since February 2016, and now they're in a sharp correction, even a bear market for some of them, and it takes time to work through."

Sohn said he's also watching industrials, including defense stocks. He's particularly watching Boeing, down about 6 percent this month.

"You have to see improvement from them. It's tough to get constructive without them working," Sohn said.

Sohn and other technical analysts are watching the 2,600 level on the S&P as a lower level of support. If it breaks that level, the S&P's next stop could be 2,350.

WATCH: Heat waves to disrupt airplanes' ability to take off