(CNN) There's another wall President Trump is building -- without spending a single cent on concrete or steel.

And a big piece of it just became official

A new measure known as the public charge rule went into effect this past Monday. It's a major change that could impact hundreds of thousands of people -- or by some estimates, even millions

The rule is effectively a wealth test for legal immigration, adding newcriteria for who can get green cards based on whether they use government benefits -- such as food stamps, Medicaid and subsidized housing -- or are expected to in the future. It's a significant step in the construction of what's become known as Trump's "invisible wall," the methodical overhaul of policies that's dramatically changing who can immigrate legally to the United States.

Like any change spelled out in forms and regulations , public charge gets complicated once you dive into the details -- and there's a lot we don't know yet about how it will play out, since it's just begun to take effect. But there's no doubt that this is a big deal.

How big?

"In terms of legal immigration, this could be the biggest change in immigration policy that we've seen in the Trump administration," said Julia Gelatt, a senior analyst at the Migration Policy Institute in Washington.

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The Trump administration argues it's just toughening up its enforcement of a law that's long been on the books. But immigrant rights groups argue there's something more sinister afoot.

"This is a fundamental rewrite of our immigration laws through executive fiat. ... The impact of this is going to be huge," said Kamal Essaheb, deputy director of the National Immigration Law Center, one of the organizations fighting the new rule in court. "This rule has the capacity to fundamentally reshape the face of America, and the face of who's coming to this country."

Here's a look at why the public charge rule is so significant, what we've already seen happening, and what could happen next.

It got a lot of attention after a top official rewrote the Statue of Liberty poem

Shortly after announcing plans for the new rule back in August, a top immigration official's comments about it caught even the Trump administration's staunchest critics by surprise.

Ken Cuccinelli -- then the acting director of US Citizenship and Immigration Services, and now the second-in-command at the Department of Homeland Security -- had been promoting the rule change as a measure that protects American taxpayers and encourages "self-reliance, industriousness and perseverance."

"Throughout our history, self-sufficiency has been a core tenet of the American dream," he wrote in a CNN opinion piece . "Long-standing federal law has required foreign nationals to rely on their own capabilities and the resources of their families, sponsors and private organizations in their communities to succeed."

Pressed to defend the policy by an NPR reporter who pointed to the words on the Statue of Liberty's pedestal -- long cited as a welcoming message for immigrants from all walks of life -- Cuccinelli offered his own spin on the iconic poem.

"Give me your tired and your poor," he said , "who can stand on their own two feet and who will not become a public charge."

Top immigration official Ken Cuccinelli drew criticism last year after he offered his own spin on the famed Emma Lazarus poem at the base of the Statue of Liberty.

Soon afterward, a series of federal court decisions blocked the public charge rule from going into effect, and for months the issue largely faded from public view once again.

But a 5-4 Supreme Court decision last month cleared the way for the controversial policy change to begin. And now, it's back in the limelight.

It's not the only brick in Trump's 'invisible wall'

The public charge rule is a major piece of the Trump administration's efforts to use bureaucratic policy changes and executive actions to reshape legal immigration in the United States. But it's far from the only one.

In the past six months alone, the administration:

to send migrants there instead. • Made it nearly impossible for migrants from Central America to seek asylum in the United States by making a series of deals with Mexico and Central American countriesto send migrants there instead.

• Reduced the number of refugees allowed to come to the United States to a historically low number

But immigrant rights advocates, who are certainly never shy about decrying the Trump administration's moves, say the public charge rule is even more severe than other measures we've seen.

"This is not a paper cut. This is a shiv to the heart of the country. ... There's nothing subtle about this," says Ali Noorani, executive director of the National Immigration Forum, an immigrant advocacy organization.

"The President is very happy to say he's in favor of legal immigration, but there's really no evidence of that. Policy after policy, regulation after regulation, block by block, the administration is slashing legal immigration to the United States."

Noorani argues that in addition to the direct impact the policy will have on hundreds of thousands of people, the public charge rule is harmful because it perpetuates a false narrative that immigrants are a drain on society. And he cautions that calling public charge part of an "invisible" policy wall underplays how dramatically the landscape is changing -- especially for immigrants and their families.

"If you're somebody who's going through the process, you see that wall. It's not invisible in any way, shape or form to you," he says.

Advocates say they're already seeing a 'chilling effect'

Immigrant rights and public health advocates have been warning for months that they're already seeing troubling signs of the new measure's impact. Legal immigrants and their US citizen children, they say, started pulling out of public programs such as Medicaid as rumors about what could be included the public charge regulation spread . Even though the final rule the government published includes more exceptions and is less severe than earlier drafts of the measure, advocates say the damage has already been done.

"The reality is for the vast majority of those people, they're not going to be adversely affected by this rule, but the chaos and fear that this rule and the broader environment that this administration is creating...all these things are having an adverse effect and will continue to have an adverse effect on the lives of children and families," Essaheb said.

California Gov. Gavin Newsom speaks at an August news conference announcing his state's lawsuit against the Trump administration over the public charge rule.

Advocates for immigrants say they're watching closely to see what happens -- and are ready to keep fighting the rule in court.

It's hard to predict exactly how many people will be directly affected by the policy now that it's in effect, because it's largely subject to the discretion of the officer who will weigh whether someone is likely to become a public charge by assessing "negative" and "positive" factors outlined in the rule.

"No one factor alone will decide an applicant's case," Cuccinelli wrote. Use of benefits like public housing, food stamps and Medicaid will generally be considered as "a negative factor," he said. Officers, he said, will also assess factors like age, health, family status, assets, resources, financial status, education and skills.

The Department of Homeland Security has said the rule is expected to impact roughly 382,000 people seeking to adjust their immigration status.

"It seems to give incredible discretionary power to USCIS application adjudicators to decide how they weigh up all of the positive and negative factors," says Gelatt of the Migration Policy Institute. "We don't know how it will play out. ... Certainly a large portion of immigrants have factors that would count against them in the rule."

Study after study has predicted the impact will be significant, especially taking into account the ripple effects of widespread fear and confusion.

"It could limit immigration severely from certain parts of the world. Immigrants from Mexico and Central America we think will have a harder time passing this public charge test than, say, immigrants from Europe," Gelatt says. "That could really reshape immigration to the United States, and potentially reduce immigration to the United States by quite a bit."