Since July, all but the most affluent families across the country have been receiving child tax credits as monthly cash payments — a first-of-its-kind policy jujitsu that converted a tax break, usually given out as a lump sum at the end of the financial year, into an additional income that expanded America’s safety net.

The program represented the first significant shift of American government support for families in decades. Since the 1990s, the child tax credit was available only to parents who were actively working or looking for work, making the United States an outlier among other developed countries where subsidies for children are common.

President Biden’s $1.9 trillion American Rescue Plan, in addition to refashioning the tax credit into monthly checks, expanded the total amount parents and other caregivers received and stripped it of any work-related conditions, making more money available to more households.

The initiative, which was initially set to run for six months, has now wound down with the last of the checks sent out on Dec. 15.

Democrats had hoped to make the tax credits permanent as part of Mr. Biden’s Build Back Better plan. But this week, the administration’s vast domestic agenda was effectively blocked, at least for now, by the centrist Senator Joe Manchin III, Democrat of West Virginia, who voted for the child tax credit expansion when it was first introduced but did not support its extension.

“I cannot vote to continue with this piece of legislation,” Mr. Manchin said on “Fox News Sunday,” citing concerns that the Build Back Better plan would add to the national debt and rising inflation.

The argument echoed that of his Republican colleagues — Senator Lindsey Graham, the senior Senate Budget Committee Republican from South Carolina, described the administration’s plan as an “inflation bomb” — who were united in their opposition to the Build Back Better legislation.

Experts have noted that since the payments of up to $300 per child started landing in bank accounts, alongside other Covid-related relief, child poverty dropped to record lows.

In an October report by the Census Bureau, around half of the roughly 300,000 recipients surveyed reported using the money on food — an indication that the tax credit was also helping to bring down hunger and food insufficiency. Many recipients also reported spending the funds on child care and school supplies.

Interviews with four families revealed other day-to-day expenses they used the funds for — from doctor’s appointments to car repairs — and the joys of a little breathing room for households that may otherwise live paycheck to paycheck.