Watch Art Cashin: Why bulls are 'on the defensive'

Art Cashin says stock market bulls are going to be "on the defensive" for the next few weeks after the S&P 500 stock index moved decisively below a key support level of 1693-1695.

Cashin, UBS' director of floor operations at the NYSE, told CNBC's Mary Thompson around midday that the S&P is vulnerable to a drop back to 1660 after breaking through the "neckline" of a technical "head and shoulders" pattern.

He also warns that the market is nervous about the last day of the third quarter on Monday amid fears Congress won't come up with a way to avoid a partial government shutdown on Tuesday. "Monday is a very critical day."