Interviews with more than two dozen hedge-fund managers, private-equity and bank officials, analysts and lobbyists made clear that Ms. Warren has stirred more alarm than any other Democratic candidate. (Senator Bernie Sanders, who describes himself as a socialist, is also feared, but is considered less likely to capture the nomination.)

Wall Street has long tried to influence American politics and generally donated to both parties, though it traditionally has been more aligned with Republicans. But while there’s no coordinated strategy, the industry is more or less united against Ms. Warren. With just months before the first voting begins, it is unleashing a barrage of public attacks, donating money to her rivals and scrambling to counter her blistering narrative about Wall Street.

“Everyone is nervous,” said Steven Rattner, a prominent Democratic donor who manages the wealth of Michael R. Bloomberg, the former New York City mayor. “What scares the hell out of me is the way she would fundamentally change our free-enterprise system.”…

Yet even if Ms. Warren cannot get some of her more far-reaching proposals through Congress, finance executives fear that as president she would appoint regulators who take a far stricter view of the industry.