Calling for the repeal of Gramm-Leach-Bliley, the legislation that in 1999 took Glass-Steagall off the books, is politically expedient on a number of levels for Trump, one banker said.
For one, the legislation was signed into law by President Bill Clinton, and the narrative gives Trump further opportunity to attack Hillary Clinton, who will run opposite Trump representing the Democratic Party after next week's convention in Philadelphia. The presumptive Democratic nominee has come under fire for her relationship with Wall Street, notably for speeches she gave at Goldman Sachs, the contents of which have yet to be fully disclosed.
Further, Trump's willingness to take on Wall Street could resonate with disenfranchised Bernie Sanders supporters who are now on the fence, another Wall Street pro noted.
"[I]t seems a bit suspicious that he rolled it out now, [it's] clearly not something he's given much thought to before," said one source at a big bank who spoke to CNBC on the condition that his name and his company not be disclosed.
Trump has obfuscated much about his future policies, but the shift toward more aggressive regulation of Wall Street could hamper his fundraising efforts, which still have a long way to go in terms of shoring up cash to support his candidacy. It could also hurt the efforts of other Republicans.
The Trump campaign has pushed to get more support on Wall Street and from the financial services industry as it looks to take on the Clinton juggernaut, expected to be armed with $1 billion to advertise in key swing states.
For all of Wall Street's surprise, however, a common link between supporters and detractors of the Trump campaign was their expectations for legislation that would simultaneously tear down Dodd-Frank, and reimplement Glass-Steagall. While the prospect of legislation to that effect becoming reality is intimidating, finance pros think other checks and balances within the U.S. government will head off the GOP candidate's pledge to rearrange Wall Street.
"No one is actually worried this will become law," one banker said.