The congressional agreement on Sunday on another dose of aid to fuel the slowing economic recovery has probably spared millions of Americans from a winter of poverty and kept the country from falling back into recession.

For much of the economy — especially people and industries that have been insulated from the worst effects of the pandemic — it may provide a bridge to a vaccine-fueled rebound. That is especially likely if the vaccine is quickly and widely distributed, and the swelling number of coronavirus cases doesn’t force another round of widespread shutdowns.

The injection of money comes months too late for tens of thousands of failed businesses, however, and it may not be enough to sustain unemployed workers until the labor market rebounds. Moreover, it could be the last help from Washington the economy gets anytime soon.

The package requires a vote in both houses, and its text was still being finalized on Sunday. But it is expected to include most of the elements that economists have long said were crucial to avoiding further calamity and aiding a recovery. It extends unemployment benefits for millions at risk of losing them, and adds money to their checks to help pay their bills. It revives the Paycheck Protection Program, which kept many small businesses afloat last spring.

It continues the eviction moratorium and expanded nutrition benefits that have kept many of the most vulnerable families fed and housed during the crisis, according to a statement on Sunday evening from the Democratic leaders in the House and the Senate.

It also provides a new round of direct payments to most Americans. That element was a lower priority for many economists, since many families have maintained their jobs and income through the highly unequal rebound from the shutdowns of the spring. Still, the checks will inject billions of dollars into the economy and will help people who have kept jobs but lost hours or income.

But the aid may not be sufficient to propel the economy beyond the kind of grinding rebound that followed recent recessions. Already, there are signs that the crisis is leaving a lasting economic toll: Long-term joblessness is rising, racial gaps are widening and more people — particularly women — are leaving the labor force.

The cash payments in the new package — up to $600 a person for households and a $300 weekly supplement to unemployment benefits — are half the size of what Congress provided last spring. That means they will provide less of an economic jolt, and won’t do as much to help replenish the savings of jobless workers getting by on benefits that typically total a few hundred dollars a week.

And two programs — one for those not covered by traditional unemployment insurance, and another that provides aid after state benefits expire — will be extended for less than three months. So millions of jobless Americans will lose crucial support if hiring does not pick up significantly in the meantime.

The recovery may also be hurt by what Congress chose not to do. Looming largest is negotiators’ inability to reach agreement on hundreds of billions of dollars to patch holes in state and local budgets that have cost 1.3 million jobs since March. Forecasters say the shortfall in revenue makes continuing layoffs likely.

“Things are not as bad as they looked in the dark days of March and April, but there still are risks,” said Tracy Gordon, a senior fellow at the Urban Institute in Washington. “It takes a while for things going on in the economy to wend their way into state budgets.”

President-elect Joseph R. Biden Jr. and congressional Democrats have characterized the aid package as a down payment to avoid short-term economic harm, an effort that should be followed by further aid to ensure a robust recovery.

But Republican opposition — and rising optimism that vaccine deployment could begin to arrest the pandemic and kick-start tourism, live events, indoor dining and other slumping industries early in the new year — makes it likely that Congress will have a hard time passing another large aid package. Achieving that goal in Mr. Biden’s early days as president could hinge on whether Democrats win two runoff elections in Georgia that will determine control of the Senate.

Lawmakers reached quick agreement on the $2.2 trillion CARES Act in March, but they were deadlocked for months on a second round of relief after the Democratic-controlled House passed a $3 trillion version in May. The delay took a toll on the recovery, hurting both households and business owners.

The recovery got off to a fast start when businesses began to reopen in May and June, but it has slowed sharply, and in recent weeks there have been signs that it is going into reverse. Layoffs are rising, retail sales are falling and the surge in virus cases has led many states to reimpose restrictions on business and consumer activity.

Data from business owners collected by Alignable, an online network for small businesses, showed steady improvement in their operations over the summer as the economy reopened — and then renewed distress since September as aid dried up, virus cases rose and consumers pulled back.

“A lot of these businesses that thought they saw the light at the end of the tunnel in June or July are now looking back and realizing it was just a train heading at them,” said Eric Groves, Alignable’s chief executive.

An analysis of 40,000 small businesses tracked by Homebase, which provides scheduling and time tracking software for businesses, shows that nearly half of companies that shut down in March, at the dawn of the pandemic, either did not reopen or reopened but then shut down again. The smallest businesses were the most likely to stay closed or close again, said Jesse Rothstein of the University of California, Berkeley, who is on the team of economists that studied the data.

“Everybody laid off a few workers” when demand plunged in the crisis, Mr. Rothstein said. “If you only had a few workers, that meant you went away.”

For the businesses that survived, the new aid package revives the Paycheck Protection Program, which offers forgivable loans to employers.

But it isn’t clear whether the aid will come in time or be sufficient to save businesses that have been pushed to the brink, said Kenan Fikri, director of research at the Economic Innovation Group in Washington.

“Small businesses have just been getting by, and now we’re entering a precarious phase where many of them cannot expect a full return in revenues for six months at least, depending on when we roll out a vaccine,” he said. “‘Did we lose in the seventh inning?’ is I guess the question we’ll find out here.”

There are reasons for optimism. The economy has proved more resilient than many forecasters expected earlier this year. The unemployment rate fell to 6.7 percent in November from a high of nearly 15 percent in April, and economists, including those at the Fed, have repeatedly raised their economic projections. Many businesses have found new ways to operate; the recent increase in layoffs is far less severe than the job losses in the spring.

That resilience is partly a result of earlier rounds of government aid, which proved to have lasting benefits. Household savings swelled in the spring when stimulus checks and enhanced unemployment benefits began appearing in Americans’ bank accounts, and while they have since fallen, the typical family’s checking account balance in October remained above pre-pandemic levels, according to data from the JPMorgan Chase Institute.

But the effects have not been evenly spread — and even if the latest round of relief helps achieve a full recovery, scars will remain.

“I don’t think we can reverse the damage,” said Michelle Holder, an economist at John Jay College of Criminal Justice in New York. “The damage is done.”

Account balances have fallen fastest for low-wage workers, who have been hit hardest by job losses during the pandemic and who were most likely to rely on the $600 federal benefit supplement that ended in July.

Researchers estimate that millions of families have slipped into poverty during the pandemic. While a new round of government aid could lift many of them back above the poverty line, they say, there will still be lasting effects.

“The best-case scenario is we look back on this and say, ‘Well, an ounce of prevention would have been worth a pound of cure,’” said Elizabeth Ananat, an economist at Barnard College who has studied the effects of the pandemic on low-income households.

“The more likely scenario,” she added, “is that we all spend the next 30 years documenting all the harm that was done because of this.”

Emily Cochrane contributed reporting.