A whole range of pandemic aid programs are set to expire in the new year, leaving millions of Americans without the government support that's helped keep them afloat—and threatening to hold back a rebounding economy.

The biggest blow will likely come from the end of two federal unemployment-insurance programs, with roughly 12 million people facing a late-December cutoff, according to a study released Wednesday by The Century Foundation. Also, measures that froze student-loan payments, offered mortgage forbearance, and halted evictions have a year-end deadline. So do Federal Reserve lending facilities for small businesses and local governments.

Some Covid-19 assistance could potentially be attached to a spending bill needed to avoid a federal government shutdown, but with Congress deadlocked and a White House transition looming, the outlook for another stimulus package this year is bleak.