The Trump administration is proposing a massive overhaul of federal regulations that would dramatically change the way the U.S. decides which immigrants are deemed a "public charge."

That new definition – if it goes into effect – would impact the applications of hundreds of thousands of immigrants trying to become legal permanent residents, the first step toward eventually becoming a U.S. citizen.

Secretary of Homeland Security Kirstjen Nielsen said in statementthat the administration was enforcing a longstanding federal law that requires immigrants to the U.S. "show they can support themselves financially."

"This proposed rule will implement a law passed by Congress intended to promote immigrant self-sufficiency and protect finite resources by ensuring that they are not likely to become burdens on American taxpayers," she said.

Critics of the proposal say it goes way too far, punishing poorer legal immigrants for receiving even small amounts of government aid just when they need it the most.

"No longer would the U.S. be a beacon for the world’s dreamers and strivers. Instead, America’s doors would be open only to the highest bidder," read a letter signed by more than 1,100 advocacy organizations opposing the proposed regulations.

More:Immigrants baffled over Trump's planned green card rules

The administration estimates the change will affect about 380,000 applications annually, but immigration advocacy groups believe it will affect far more. The U.S. grants green cards to roughly 1 million foreigners each year, and advocates worry the new regulations could complicate a far higher number of those applications.

Lawyers across the country have been digesting the proposed changes, so here are answers to the most pressing questions over the proposed overhaul.

What is a 'public charge'?

The idea that immigrants to America should be self-sufficient predates the Constitution, when individual colonies passed laws to ensure that "rogues," "vagabonds," and "undesirables" did not enter their territories.

The first time it was codified into U.S. law was the Immigration Act of 1882, which instructed local port officers to turn back any "convict, lunatic, idiot, or any person unable to take care of himself or herself without becoming a public charge."

The term was further clarified in the 1990's when President Bill Clinton signed a welfare overhaul into law and the now-defunct Immigration and Naturalization Service, or INS, issued field guidance to instruct its officers how to apply it. The guidance, which largely remains in effect, defines a public charge as someone who is "primarily dependent" on government assistance. That means receiving cash assistance that makes up more than half of their income, or receiving long-term medical care "at government expense."

How is the definition changing?

The new regulations proposed by the Trump administration involve a complicated formula that vastly expands the definition of public charge to include any immigrant who receives varying levels of government aid.

That includes cash programs scrutinized by previous administrations, such as Supplemental Security Income (SSI), Temporary Assistance for Needy Families (TANF), and state and local cash assistance. The new regulations would also consider "non-cash" benefits, such as Supplemental Nutrition Assistance Program (known as food stamps), Section 8 housing and rental assistance, Medicare Part D prescription drug benefits, and Medicaid in non-emergency situations.

The new rules allow immigration officials to weigh a wide variety of "negative factors" that the administration considers an indication that the person is likely to become a public charge, but which immigration advocates say is far too broad and penalizes children, the elderly, and the poor. Those factors include the applicants' age (specifically if an applicant is under 18 or over 61), health, education, work skills, income, and family status.

The new limits would apply to anybody seeking a green card, and many foreigners trying to get, or extend, temporary visas.

The administration says the changes would save U.S. taxpayers $2.7 billion a year by denying immigrants who are considered likely to receive government benefits. But advocacy groups, including the Center for Law and Social Policy and the National Immigration Law Center, say the new rules would discriminate against low-skilled, low-income immigrants, transforming the U.S. "from a country that welcomes people who plan to work hard and achieve a better life, to one rigged in favor of the wealthy."

How often has it been used?

Washington's focus on the financial viability of immigrants has varied over the centuries.

From 1890 to 1920, a period of mass migration from Europe, up to 70 percent of green card denials were based on public charge grounds. But according to a report by the non-partisan Migration Policy Institute, that percentage plummeted to 4 percent by the 1940s and 1 percent by the 1950s.

More recently, the practice of denying immigration based on public charge grounds has fluctuated. Used often throughout the 1990s, it plummeted during the tenure of GOP President George W. Bush, who opposed restricting immigration to the U.S. In 2000, U.S. consular officials turned down 46,450 applications for green cards, according to State Department data. By 2008, Bush's final year in office, that number had fallen to 6,852. The downward trend continued under President Barack Obama, with 1,076 public charge denials by consular officials in his final year in office.

It's unclear how quickly the proposed regulations could alter those numbers, but advocates worry that the change could happen overnight.

When will new rules go into effect?

The government published the 183 pages of proposed rules in the Federal Register on Wednesday. That will kick-start a 60-day period of public comment, where legislators, advocacy groups and members of the public can weigh in on the changes, so it could take months for the new rules to take effect.

The department is then expected to consider the public comments before issuing a final rule, a process that normally takes months to finalize, but could be sped up.

The other possibilities are a quick rewrite of public charge laws by Congress or a lawsuit challenging its legality. The congressional route is highly unlikely, as members have been unable to reach consensus on any aspect of immigration law in recent years. Lawmakers will be even more reluctant to take up such a controversial measure with the midterm elections less than two months away.

That leaves the legal route, which immigration and civil rights advocates have used successfully to block, or alter, many aspects of Trump's immigration policies.