Since then, officials have been pumping cash into the market for short-term loans between banks, called repurchase agreements or “repos,” to keep the market functioning smoothly. While that effort is temporary, bill purchases are a longer-lived fix. The two programs added about $400 billion to the central bank’s holdings in the second half of 2019.

As the Fed prepares to tiptoe away from bill-buying in particular, it is creating a tense moment for Wall Street. Stock prices have risen on the back of the purchases, and analysts warn that the gains could fade as buying slows.

“That will take away some of the fuel that has probably been able to help the markets out,” said Joseph Kalish, chief global macro strategist at Ned Davis Research, a market research firm.

Chair Jerome H. Powell is widely expected to field questions about the Fed’s plans at a news conference after its two-day policy meeting on Wednesday. The meeting itself is expected to produce little drama — the Fed is likely to leave rates unchanged — but Mr. Powell’s comments about the purchases could generate headlines and roil markets.

Fed officials took pains to clarify from the start that the Treasury bill purchases were simply a technical fix aimed at lifting the amount of bank reserves in the financial system. The multibillion-dollar effort was not, they insisted, the same as the “quantitative easing” bond-buying programs it periodically undertook after the financial crisis to aid economic growth.