In recent weeks, the Fed has neared consensus that its workhorse tool will be the interest it pays banks on excess reserves on deposit at the Fed – giving the central bank a direct way to encourage banks to either take money out of circulation and leave it at the Fed, or lend it elsewhere.
Another tool would have a similar impact but apply more broadly, using overnight repurchase agreements that would let money market funds and other institutions as well as banks essentially make short-term deposits at the Fed.
The worry is that if financial conditions tighten, those large funds of money would flee to the repo facility as a safe haven, depriving the economy of credit and making a potential crisis even worse.