WASHINGTON — Treasury Secretary Steven Mnuchin said he does not plan to extend several key emergency lending programs beyond the end of the year and asked the Federal Reserve to return the money supporting them, a decision that could hinder President-elect Joseph R. Biden Jr.’s ability to use the central bank’s vast powers to cushion the economic fallout from the virus.Mr. Mnuchin on Thursday said he would not continue Fed programs, including ones that support the markets for corporate bonds and municipal debt and one that extends loans to mid-sized businesses. The emergency efforts expire at the end of 2020, but investors had expected some or all of them to be kept operational as the virus continues to pose economic risks.The pandemic-era programs are run by the Fed but use Treasury money to insure against losses. They have provided an important backstop that has calmed critical markets since the coronavirus took hold in March. Removing them could leave significant corners of the financial world vulnerable to the type of volatility that cascaded through the system as virus fears mounted in the spring. By asking the Fed to return unused funds, Mr. Mnuchin could prevent Mr. Biden’s incoming Treasury secretary from quickly restarting the efforts at scale in 2021.“The Federal Reserve would prefer that the full suite of emergency facilities established during the coronavirus pandemic continue to serve their important role as a backstop for our still-strained and vulnerable economy,” the central bank said in a statement.The emergency programs were backed by $454 billion that Congress appropriated in March as part of a broader pandemic response package. Because of the way the Fed’s emergency lending powers work, Jerome H. Powell, the Fed chair, needs the Treasury secretary’s signoff to make major changes to the programs’ terms. Extending the end date counts as one of those changes that need approval.The decision to close the various programs and remove the funding appeared to come as a surprise to the Fed, which received a letter announcing the Treasury’s desire to claw the money back on Thursday afternoon. “I am requesting that the Federal Reserve return the unused funds to the Treasury,” Mr. Mnuchin said in the letter. He noted that he had been “personally involved in drafting the relevant part of the legislation” and believed it was Congress’s intent that the programs stop at the end of the year. Earlier this month, Mr. Powell had said the central bank and Treasury were just beginning to discuss whether to extend the programs.Mr. Mnuchin did agree to extend other emergency loan programs that are not backed by the congressional appropriation, including ones that service the short-term market for corporate debt, one for money market funds, and one that backstops government small-business loans.The Fed avoids taking credit losses when extending loans, and throughout the pandemic crisis it has asked for Treasury backup for its riskier programs. If it returns any unused money that the Treasury has already dedicated to support the programs, as Mr. Mnuchin requested, the Biden administration will have less financial backup to restart the programs.That’s because the congressional appropriation — $195 billion of which has been earmarked to specific Fed programs — cannot be used to make new loans after the end of the year. But while the law prohibits the Treasury from putting money into the Fed’s facilities after 2020, it does not obviously prevent the Fed from using already-earmarked Treasury funding to insure its own loans and bond purchases. “The loans, loan guarantees and investing that the Treasury does is the applicable language,” said Peter Conti-Brown, a lawyer and Fed historian at the University of Pennsylvania. He said that while it may be possible to read the law as preventing new Fed loans, that is not the “obvious reading.” The Fed and the next Treasury secretary do have an alternative to continue the programs: They could use money in the Treasury’s Exchange Stabilization Fund, which still contains about $74 billion in uncommitted funds, to back the programs. It is unclear exactly how much of the fund can be used, but the programs have not to date needed substantial capacity. Mr. Mnuchin’s move could leave the government with fewer options to help the economy just as the new administration takes office. “Treasury is right that a limited set of objectives have been achieved in terms of stabilizing bond markets,” Jason Furman, a prominent Democratic economist, said on Twitter. “But what is the downside to continuing them as insurance against worse developments?” Many of the Fed’s programs, including one that buys state and local debt and another that encourages banks to lend to small- and mid-sized businesses, have been lightly used. But that is because they were designed as backstops — meaning that borrowers would likely only use them when times are bad. And it is Mr. Mnuchin himself who has been conservative in setting the program’s terms. With a more permissive head at the Treasury, the terms could have been made more generous. In fact, Democrats had been eyeing both the municipal bond-buying program and the Main Street lending effort for small- and medium-sized businesses as potential backup options if it proves difficult to pass additional government relief. Without them, businesses and state and local governments would have one less potential source of help. With coronavirus cases on the rise, the economy may sour again, making the programs more necessary. As recently as Tuesday, Mr. Powell warned of the potential for economic scarring and said that the economic recovery had “a long way to go.” But Treasury officials have expressed optimism that the economy is poised for a steady rebound and that the likely rollout of a vaccine by the end of the year further improves the economic picture. Senator Patrick J. Toomey, Republican of Pennsylvania, who had been pushing Mr. Mnuchin to end the programs, applauded the decision.“These temporary facilities helped to both normalize markets and produce record levels of liquidity,” Mr. Toomey said in a statement. “Congress’s intent was clear: These facilities were to be temporary, to provide liquidity, and to cease operations by the end of 2020.”Treasury’s move prompted concern from Democrats, some of whom said the Fed should simply refuse to return the money — a route it is unlikely to take. Bharat Ramamurti, a Democrat who sits on the congressional oversight body in charge of reviewing the various Fed and Treasury programs, suggested on Twitter that, legally, the Fed was under no obligation to give back the funds.“Under its contracts with Treasury, the Fed can and should reject the request,” he said. “While Secretary Mnuchin claims congressional intent was to halt all new loans at year-end, the text of the CARES Act doesn’t say that. At a minimum, the Fed can continue to make loans using the $195 billion in equity Treasury has already committed.”— Emily Cochrane contributed reporting.