"I'm not sure that I see much distinction between what you're doing now and what the Fed was doing in pre-2008, and I think that's deeply worrisome," she said. "I'm very concerned that the Fed dropped the ball before and they may be dropping it one more time."
Leveraged lending actually has declined in 2018 from its blistering pace the year before.
Total volume in the third quarter hit $177 billion, a 40 percent slide compared with the record pace of the same period in 2017, according to Thomson Reuters. Issuance so far this year is tracking at $930 billion, which is a 12 percent decline from a year ago.
Institutional share of leveraged loans edged lower to 58 percent in the third quarter, against 60 percent from 2017.
Still, Warren said she sees echoes in the current situation from what happened before the crisis, when subprime loans to underqualified borrowers helped tank the financial system. The senator outlined her concerns in a letter to multiple regulators: Treasury Secretary Steven Mnuchin; Comptroller of the Currency Joseph Otting; Fed Chairman Jerome Powell, Securities and Exchange Commission Chairman Jay Clayton and FDIC Chair Jelena McWilliams.
She challenged Quarles about whether the Fed is keeping a watch over bank leverage ratio and the standards institutions are using when approving loans.
"The Fed dropped the ball before the 2008 crisis by ignoring the risks in the subprime mortgage market," Warren sad. "What are you doing differently this time in coordination with other federal regulators so that you're limiting the risk that leveraged loans cause serious harm to the financial system?"
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