As markets melted down in March, the Federal Reserve unveiled novel programs meant to keep credit flowing to states, medium-sized businesses and big companies — and Congress handed Treasury Secretary Steven Mnuchin $454 billion to back up the effort.

Nine months later, Senate Republicans are trying to make sure that those same programs cannot be restarted after Mr. Mnuchin lets them end on Dec. 31. Beyond preventing their reincarnation under the Biden administration, Republicans are seeking to insert language into a pandemic stimulus package that would limit the Fed’s powers going forward, potentially keeping it from lending to businesses and municipalities in future crises.

The last-minute move has drawn Democratic ire, and it has imperiled the fate of relief legislation that economists say is sorely needed as households and businesses stare down a dark pandemic winter. Here is a rundown of how the Fed’s lending powers work and how Republicans are seeking to change them.

The Fed can keep credit flowing when conditions are really bad.

The Fed’s main and best-known job is setting interest rates to guide the economy. But the central bank was set up in 1913 in large part to stave off bank problems and financial panics — when people become nervous about the future and rush to withdraw their money from bank accounts and sell off stocks, bonds and other investments. Congress dramatically expanded the Fed’s powers to fight panics during the Great Depression, adding Section 13-3 to the Federal Reserve Act.

The section allows the Fed to act as a lender of last resort during “unusual and exigent” circumstances — in short, when markets are not working normally because investors are exceptionally worried. The central bank used those powers extensively during the 2008 crisis, including to support politically unpopular bailouts of financial firms. Congress subsequently amended the Fed’s powers so that it would need Treasury’s blessing to roll out new emergency loan programs or to materially change existing ones.

The programs provide confidence as much as credit.

During the 2008 crisis, the Fed served primarily as a true lender of last resort — it mostly backed up the various financial markets by offering to step in if conditions got really bad.

The 2020 emergency loan programs have been way more expansive. Last time, the Fed concentrated on parts of Wall Street most Americans know little about like the commercial paper market and primary dealers. This time, it reintroduced those measures, but it also unveiled new programs that have kept credit available in virtually every part of the economy. It has offered to buy municipal bonds, supported bank lending to small and medium-sized businesses, and bought up corporate debt.

The sweeping package was a response to a real problem: Many markets were crashing in March. And the new programs generally worked. While the terms weren’t super generous and relatively few companies and state and local borrowers have taken advantage of these new programs, their existence gave investors confidence that the central bank would prevent a financial collapse.

But things started getting messy in mid-November.

Most lawmakers agreed that the Fed and Treasury had done a good job reopening credit markets and protecting the economy. But Senator Patrick J. Toomey, a Pennsylvania Republican, started to ask questions this summer about when the programs would end. He said he was worried that the Fed might overstep its boundaries and replace private lenders.

After the election, other Republicans joined Mr. Toomey’s push to end the programs. Mr. Mnuchin announced on Nov. 19 that he believed Congress had intended for the five programs backed by the $454 billion Congress authorized to stop lending and buying bonds on Dec. 31. He closed them — while leaving a handful of mostly older programs open — and asked the Fed to return the money he had lent to the central bank.

The Fed issued a statement saying it was dissatisfied with his choice, but agreed to give the money back.

Democrats criticized the move as designed to limit the incoming Biden administration’s options. They began to discuss whether they could reclaim the funds and restart the programs once Mr. Biden took office and his Treasury secretary was confirmed, since Mr. Mnuchin’s decision to close them and claw back the funds rested on dubious legal ground.

The new Republican move would cut off that option. Legislative language circulating early Friday suggested that it would prevent “any program or facility that is similar to any program or facility established” using the 2020 appropriation. While that would still allow the Fed to provide liquidity to Wall Street during a crisis, it could seriously limit the central bank’s freedom to lend to businesses, states and localities well into the future.

In a statement, Senator Elizabeth Warren, Democrat of Massachusetts, called it an attempt to “to sabotage President Biden and our nation’s economy.”

Mr. Toomey has defended his proposal as an effort to protect the Fed from politicization. For example, he said Democrats might try to make the Fed’s programs much more generous to states and local governments.

The Treasury secretary would need to have the Fed’s approval to improve the terms to help favored borrowers. But the central bank might not readily agree, as it has generally approached its powers cautiously to avoid attracting political scrutiny and to maintain its status as a nonpartisan institution.

Fed officials have avoided weighing in on the congressional showdown underway.

“I won’t have anything to say on that beyond what we have already said — that Secretary Mnuchin, as Treasury secretary, would like for the programs to end as of Dec. 31” and that the Fed will give back the money as asked, Richard H. Clarida, the vice chairman of the Fed, said Friday on CNBC.

More generally, he added that “we do believe that the 13-3 facilities” have been “very valuable.”

Emily Cochrane contributed reporting.