The behavior of the Russell 2000 is also being taken as a warning, and even the Fed has picked on the index, saying it is overvalued. The Russell has been underperforming the broader market, and it is down nearly 8 percent in the past four weeks, compared with the 2.6 percent decline in the S&P 500.
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"The Russell's been the weakest of them all," LaRosa said. "I think the next major level is going to be 1,080. We have to defend that level, which we've hit a couple of times. If we close below that, we could have significant problems."
LaRosa said 1,131 on the Russell was a key number and that was broken last week. The market breadth has also been weakening.
"I'm not surprised we're getting selling. This market for several months now didn't have the characteristics of one that you would expect to continue to rise...When the market is hitting new highs and you have that backdrop, a correction is more likely and for the last three or four sessions, you're really seeing that."
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Curry said the Russell has been signaling problems for awhile. "The Russell's been in a big range while the S&P 500 and Nasdaq had been making new highs. The Russell has not, so the divergence is a concern," he said.
Technicians say the Russell could test the key level around 1,080. The Russell was trading at 1,119 Monday, up 0.4 percent.
Laszlo Birinyi, stock strategist and founder of Birinyi Associates, disputes the idea that the Russell is a warning for the broader market, or a harbinger for the economy.
"Our view continues that this is an issue primarily for those shares and small cap portfolios and has limited implications for the broader market as a whole," he wrote in a note Monday.
Another area of concern is the high-yield market, and the selloff that has been going on for six weeks, Curry said. "Credit in general was not confirming the price action in the S&P 500 which is often a precursor to a corrective environment," he said.
Worth has been negative for stocks for awhile, said the market's weakness has been masked by the outperformance of "a handful of super captialization stocks."