(Bloomberg) -- Consumer advocacy groups and labor unions are lining up to support plans by the FDIC to step up scrutiny of stakes in US banks held by money managers such as BlackRock Inc. and Vanguard Group Inc., citing the danger of too much concentration.

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The groups made their case in a letter sent to the Federal Deposit Insurance Corp. late Tuesday that warned of the investment firms’ growing influence over corporate decisions, including at banks. They also called on officials to address potential threats to financial stability posed by massive asset managers.

“Regulators need to catch up to the reality that the growth and concentration of the asset management industry has fundamentally reshaped how public companies — including listed banks — make decisions,” said Natalia Renta, a senior policy counsel at the Americans for Financial Reform Education Fund. Her group signed the comment letter along with dozens of organizations, including the American Federation of Teachers and the AFL-CIO.

The coalition was responding to a request for comments from the FDIC, which has proposed more controls and oversight of the big money managers due to concerns that concentrated ownership could give them undue influence over lenders. The agency is charged with protecting trillions of dollars of domestic deposits at more than 4,500 banks.

BlackRock has pushed back on the FDIC’s plan, saying it would upend index funds that dominate many investor portfolios, make it more costly for banks to raise capital and disrupt the economy.

Earlier this year, the regulator proposed additional controls and oversight for money managers’ sizable stakes in banks due to concerns that concentrated ownership could give firms like BlackRock, Vanguard and State Street Corp. undue influence over lenders. The companies are among the biggest providers of so-called passive index funds, which seek to match returns of a specific market benchmark such as the S&P 500.

BlackRock didn’t immediately provide comment. State Street didn’t respond to an emailed request for comment.

“As an independent asset manager, Vanguard’s interests are squarely aligned with empowering everyday investors to reach their long-term financial goals, leaving management decisions to companies and public policy decisions to policymakers,” the company said in a statement Wednesday.