Wall Street is not pleased. A top advisor to presumptive GOP presidential nominee Donald Trump said on Monday that the party wants to reimplement Glass-Steagall, Depression-era legislation that was designed to prevent big bank "supermarkets," but which was repealed in 1999. After the surprise announcement, which came on the first day of the Republican National Convention, Wall Street sources sounded off on the idea that a Republican would reverse course on policies nearly 20 years old and now taken for granted by big banks. One lawyer, who works with financial institutions on behalf of a white-shoe firm in New York, called the idea "scary." Even Wilbur Ross, one of the Trump campaign's biggest supporters from the finance industry, called it "surprising." Others on Wall Street who spoke to CNBC used stronger language that can't be printed.

A spokeswoman for the Trump campaign did not respond to multiple requests for comment.

Traders at the New York Stock Exchange watch Donald Trump speaking on TV. Michael Nagle | Bloomberg | Getty Images

Glass-Steagall is legislation the U.S. imposed in the wake of the 1929 market crash aimed at limiting the relationships between securities firms and commercial banks, and by extension of that, systemic risk to U.S. markets and the economy. In 1999, legislation was passed that did away with Glass-Steagall, but now, the GOP is ready to bring it back and break up banks. "We support reinstating the Glass-Steagall Act of 1933 which prohibits commercial banks from engaging in high-risk investment," according to the 2016 Republican Party platform statement. In a seeming contradiction, the 2016 Republican platform simultaneously calls for the dismantling of Dodd-Frank, post-financial crisis regulation that's aimed at curtailing risk at too-big-to-fail institutions. Ross said on Tuesday that Dodd-Frank puts regulatory requirements on banks that are too onerous. Specifically, the GOP platform highlights the decline in the number of community banks in the U.S. and says Dodd-Frank is "regulatory harassment of local and regional banks," arguing that it and the Consumer Financial Protection Bureau "would leave us with just a few enormous institutions, as in many European countries."