Morgan Stanley

In letters to the Treasury secretary and CEOs of the largest banks, the Massachusetts Democrat questioned why Mnuchin was trying to quell liquidity fears that had not previously been mentioned by regulators.

There had been a cascade in recent days of downgrades and other negative forecasts in connection with big banks, but some analysts on Wednesday picked up the industry's argument that fundamentals are stronger than the markets are giving banks credit for.

Treasury Secretary Steven Mnuchin called top executives from the six largest U.S. banks over the weekend, he said Sunday on Twitter, a move that followed heavy losses in the stock market last week and a partial federal government shutdown.

A definition would give lenders a clearer idea of what practices are off limits; the mutual fund giant will provide the service for institutional clients.

To keep pace with its peers, Morgan Stanley is ramping up its online offerings in banking and wealth management.