Washington DC - Around two dozen states run by Democrats sued President Donald Trump's administration Tuesday over its refusal to tap emergency funds to preserve vital food aid threatened by the government shutdown.

Now on its 28th day, the standoff in Congress over spending is increasingly piling pain on the public sector, with the largest federal employees' union pressuring Senate Democrats to reopen the government.

Food stamps are rapidly becoming one of the most pressing pain points in the shutdown, with 42 million low-income Americans set to lose access to vital help with grocery bills from Saturday.

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The government has indicated that it won't put a $5 billion "contingency" fund towards the estimated $8 billion required to ensure the Supplemental Nutrition Assistance Program pays out November's benefits.

Officials in the Agriculture Department have argued that the rainy-day fund is for natural disasters and other unforeseeable events rather than shutdowns.

"Nearly 600,000 children in our state could be without food in a few days because USDA is playing an illegal game of shutdown politics," North Carolina Attorney General Jeff Jackson, one of the plaintiffs, said in a statement.

"They have emergency money to help feed children during this shutdown, and they're refusing to spend it. I warned them last week that I would take them to court if they tried to hurt our kids, and today that's what we're doing."

Around 18 million SNAP recipients live in states and districts that Democrat Kamala Harris won in the 2024 presidential election, according to an AFP analysis of federal data, while a much larger 23.7 million live in states that voted for Trump.

Agriculture Secretary Brooke Rollins placed the blame for the crisis squarely with Democrats, arguing that their repeated blocking of a Republican-backed funding bill to reopen the government had driven the country to "the cliff."

"I've been warning about this for almost a month now – that we have enough money to get us through the end of October, but after that, the government has to reopen," she told Fox News.

"And... that's where we are right now. It is stunning to me. I don't understand what they're thinking."

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Legislation has been introduced in Congress to keep SNAP benefits funded throughout the shutdown, but it is nowhere near being signed into law.

States have meanwhile been encouraging residents reliant on help to go to food banks if their benefits pause.

The SNAP cliff, along with federal workers and military service members going without pay, is piling pressure on lawmakers to end the shutdown, which has been grinding government functions to a halt since October 1.

The Democratic governors of 23 states and Democratic attorneys general of two further states – along with the capital Washington – are asking a federal judge in Massachusetts to overturn government directives for states to withhold benefits and to rule that officials must use all available funds to keep the food aid flowing.