(Reuters) - Middle- and low-income U.S. families now have significantly fewer liquid resources like bank deposits than they were on track to have before the disruptions of the COVID-19 pandemic, creating financial strains that pose a risk to consumer spending, the backbone of the economy.

Research published on Monday by the Federal Reserve Bank of San Francisco showed that for the top 20% of households by income, liquid assets - including cash and funds in savings, checking and money market accounts - rose sharply in 2020 into early 2021. They then dropped gradually and are now about 2% below what would have been expected without the pandemic shock.

But for the rest of American households, those liquid assets rose less sharply and the excess was depleted earlier and is now about 13% lower than the projected path prior to the pandemic. At the same time credit card delinquencies among these middle- and low-income families rose earlier, faster, and to "notably higher" rates than for high-income families, they showed.

"Smaller financial cushions and heightened credit stress for households at the bottom 80% of the income distribution pose a risk to future consumer spending growth," wrote economists Hamza Abdelrahman, Luiz Edgard Oliveira and Adam Shapiro.

Consumer spending - accounting for roughly two-thirds of U.S. economic output - and the labor market held up unexpectedly well during the Fed's 2022-2023 rate-hike campaign, bolstering optimism among policymakers that they could quell inflation without triggering a recession and sharp rise in unemployment - a rare economic "soft landing."

U.S. central bankers have said that continued strength in the real economy has given them room to hold the policy rate in its current 5.25%-5.50% range so as to keep up the downward pressure on inflation.

Recent economic data, however - including a report showing a jump in the unemployment rate to a post-pandemic high of 4.3% and a slowdown in hiring in July - have fueled fears that policy may be becoming too restrictive. Monday's research from the San Francisco Fed may add to the sense that cracks are building.

While consumer spending contributed significantly to the stronger-than-expected pace of economic growth in the second quarter, its monthly growth rate has slowed. Spending growth averaged 0.3% in the three months through June, its slowest average pace in more than a year.

Last week, Chicago Fed President Austan Goolsbee said the uptick in credit card delinquencies was among the factors he was watching as a possible sign that policy may be getting tighter than warranted.

Fed Chair Jerome Powell in July indicated that policymakers may move to cut rates as soon as next month as recent data shows inflation trending towards the Fed's 2% target.