It became clear this summer that public health measures across much of the country were relaxed too soon and without proper medical safeguards against the coronavirus. So now, once again, the commerce that Americans rely upon is retrenching. About 80% of Americans live in places that are pausing or dialing back reopening.

Yet the Senate left for its August recess without a compromise plan on a coronavirus relief bill for states, cities, the unemployed, businesses and the public health system. If senators still fail to resolve stalled negotiations when they return after Labor Day, millions of needy Americans will suffer — and the overall economy could degrade from its current slow rebound in growth to no growth at all.

Both monetary policy, which is the Federal Reserve’s job, and fiscal policy, the job of the federal government, have complementary roles to play in supporting the economy. (State governments can’t help because their revenues are plummeting and they are mandated to balance their budgets, which require spending cuts and layoffs and only add to the economy’s woes.)

The economics of this moment are not complicated: A self-sustaining recovery cannot occur unless the virus is controlled. It is true that after the first shutdowns of March and April, the economy did begin, in May and June, to pull itself out of a deep, pandemic-induced hole, thanks in part to generous $600 per week federal unemployment assistance that the Senate let expire in July after negotiations between Democrats and Republicans broke down.

Now, so-called real-time data show consumer spending slowing overall and deteriorating conditions for low-income households, who have become more anxious about how they will pay for their rent and their food. In a recent survey, 12% of American adults, or 30 million, reported that their household sometimes or often didn’t have enough food in the past week. (For Black and Latino households, the share was about 21%.)

These numbers reflect the confluence of at least three forces: acceleration of the spread of the virus; expiration of the supplemental federal unemployment benefits; and the ending of various eviction moratoriums. All three developments disproportionately affect low-income people and persons of color. And aside from the grave ethical questions raised by ending crucial safeguards for the vulnerable, such actions endanger the economy as a whole.

The Federal Reserve has largely done its job. By mid-March, it cut short-term interest rates to zero, and all but promised to keep them there for quite a long time. The Fed also bought large quantities of government bonds and government-backed mortgage securities to keep markets functioning and to keep borrowing costs low. These actions have pushed the 10-year Treasury yield down to almost its lowest level ever, which will spur more spending in crucial sectors like housing and automobiles.

In March, when the credit market, the economy’s bloodstream, began to clog, the Fed established so-called emergency “facilities” to keep credit flowing — to businesses small and large and to state and local governments. This forceful response cleared the blockages.

Congress, however, cannot expect the Fed to keep everything together on its own.

When unemployment is exceptionally high and inflation is historically low, as they both are now, the economy needs more fiscal spending to support hiring. Monetary power sets the table and Congress’s fiscal dollars bring in the diners.

In this way, they form a potent one-two punch against stagnation. The Fed makes sure the credit backdrop supports growth; Congress and the president make sure families and businesses have enough money in their pockets.

As its chair, Jerome Powell, has recently stressed, the Fed has “lending powers, not spending powers.” The Fed can’t send out checks to households, increase unemployment payments, stay evictions or provide grants to small businesses on the verge of shuttering. These are jobs for Congress and for the Trump administration.

Until August, Congress had actually been quite a strong partner to the Fed’s work. The situation now — congressional inaction in extending fiscal support — is reminiscent of a similar period after the last recession.

At the start of 2011, unemployment was still elevated at just over 9%. The Fed had lowered interest rates to around zero. But Congress allowed fiscal support to lapse, worried more about deficits than all those still unemployed. The Fed chair at the time, Ben Bernanke, summarized the problem well when he said, “With fiscal and monetary policy working in opposite directions, the recovery is weaker than it otherwise would be.”

With inflation as low as it is, servicing the debt required by the one-two punch of aggressive monetary and fiscal policies is relatively inexpensive.

So why, then, are we back here again? Why is Mr. Powell having to make the same pleas to Congress that Mr. Bernanke did and why is a Fed chair being ignored again?

We weren’t in the room, so we don’t know exactly why congressional negotiations broke down or what it will take for them to restart. But we could not be more confident that our economic prescription is the right one. The Fed stepped up. Once again, it’s Congress’s turn.

Janet Yellen, a former chair of the Federal Reserve, is a distinguished fellow at the Brookings Institution. Jared Bernstein is a senior fellow at the Center on Budget and Policy Priorities.