"After several months of moving sideways, a break of support at $76 support indicates that the stock's downtrend is resuming," Wald wrote to CNBC. "We see downside to $69."
That would represent a further decline of 8 percent. And if American Express shares were to drop by that amount, it would shave 38 points off of the Dow.
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But it's not just American Express that appears vulnerable. Looking at the Dow more generally, the technician believes the recent market weakness is likely to continue.
"Stocks in the Dow Jones industrial average continue to break down one by one, and many of these stocks should remain under pressure," Wald wrote to CNBC.
When it comes to the index itself, "we believe this expanding list of breakdowns could lead to a test of 15,650 support in the coming weeks."