Republican frontrunner Donald Trump said Sunday he would overturn a law that grants citizenship to people born in the U.S. and put stricter limits on legal immigration, offering his most detailed account yet of how he would handle a policy issue that has become a cornerstone of his campaign.

Trump, who has repeatedly been pressed for specifics on his immigration plan since the issue rocketed him to the top of the polls, also explained for the first time how he will force Mexico to pay for a wall on its border with the U.S. in his nearly 1,900-word policy paper.

The proposal could help Trump swat away naysayers who charge that he is not a serious candidate. It also gives Trump an opportunity to burnish his conservative credentials, particularly as he is coming under more heavy fire from conservative influencers.

Trump took a shot at his Republican opponent Sen. Marco Rubio of Florida, slamming what he calls the “Schumer-Rubio” comprehensive immigration bill, which passed the Senate in 2013 and later died in the House, as “nothing more than a giveaway to the corporate patrons who run both parties.”

Trump’s immigration plan is based on three core principles: that the U.S. must build a wall across the U.S.-Mexico border, that immigration laws must be fully enforced and that “any immigration plan must improve jobs, wages and security for all Americans.”

His policy mixes some long-held Republican proposals on immigration with ideas that are more likely to appeal to the far right.

Trump calls for requiring a nationwide system to verify workers’ legal status, tripling the number of immigrations and customs enforcement agents and implementing a tracking system to identify people who overstay their visas.

But Trump’s plans take a hardline approach in his vow to reverse a U.S. law that grants American citizenship to any child born in the United States, regardless of whether the child’s parents are undocumented immigrants.

He also calls for suspending the issuance of any new green cards, writing, “there will be a pause where employers will have to hire from the domestic pool of unemployed immigrant and native workers.”

Trump’s policy proposal does not explain how long the pause will last.

Even Trump’s approach to the Dreamers — those who were brought to the U.S. illegally as children — goes a step further than others in the GOP field who believe children of undocumented immigrants should be allowed to stay in the U.S.

“The executive order gets rescinded,” Trump said Sunday on NBC’s “Meet the Press,” of President Barack Obama’s executive order allowing dreamers to remain in the U.S.

“We have to keep the families together, but they have to go,” he told NBC.

While Trump has called for deporting all of the undocumented immigrants in the United States and allowing “the good ones,” to re-enter legally, his policy outline makes no mention of that plan. Instead, it calls for deporting all “criminal aliens.” It does not address the deportation of otherwise law-abiding undocumented immigrants.

Trump also explained how he would force the Mexican government to bankroll a wall along the southern border.

If Mexico refuses to pay for the wall, a Trump administration would begin charging additional fees to Mexicans who come into the U.S. on visas or with border crossing cars — particularly for visas to “Mexican CEOs and diplomats,” which Trump would cancel “if necessary.” Trump’s plan also calls for possible tariffs and foreign aid cuts and would seize “all remittance payments derived from illegal wages.”

“The Mexican government has taken the United States to the cleaners. They are responsible for this problem, and they must help pay to clean it up,” Trump wrote. “We will not be taken advantage of anymore.”

Immigration advocates quickly slammed Trump’s proposal Sunday.

“The extremism is stunning, and the direction is dangerous,” said Frank Sharry, the executive director of pro-immigration group America’s Voice. “Trump’s ‘plan’ would create a police state capable of rounding up and deporting all 11 million hardworking immigrant families settled in America.”

Sharry also criticized Trump’s move to change the birthright citizenship rules and put further limits on legal immigration.

“Fortunately, these marginalized ideas are as unpopular as they are unworkable, and thus will never happen,” he said.

Trump’s policy plan drew praise, though, from the Federation for American Immigration Reform, a group that supports reducing the number of both legal and undocumented immigrants in the U.S. and has been labeled a hate group in 2007 by the Southern Poverty Law Center.

“In much the same way that we enforce most civil laws in this country, the plan aims at deterring most violations of the law, and provides meaningful penalties for those who are not deterred,” spokesman Ira Mehlman said in an email.

But Trump’s immigration plan would surely draw stiff opposition from Democrats in Congress, but it’s also likely to raise alarm with the big business, a major proponent of high-skilled visas.

The price tag for his plan could draw the ire of Republicans, too.

A 2013 bipartisan immigration bill that cleared the Senate called for doubling the number of border patrol agents and completing some 700 miles of fence along the border. The nonpartisan Congressional Budget Office pegged the cost of the bill around $23 billion, mainly because of the security measures included.

Ultimately, the CBO said the Senate bill would have reduced the federal deficit by legalizing millions of immigrants living in the country illegally and, in turn, boosting tax revenues. The CBO also said that legislation would spur economic growth.

Trump’s plan, however, does not include those revenue-generating provisions.

To prevent additional undocumented immigrants for entering and staying in the United States, Trump pledged to “defund sanctuary cities” — stripping cities of federal dollars if they do not fully cooperate with federal immigration officials — establish a nationwide employment “e-verify” system, and end the “catch-and-release” policy of detaining illegal immigrants without deporting them.

While most of Trump’s brash rhetoric has focused on ending illegal immigration peppered with charges that immigrants coming from Mexico are “killers” and “rapists,” Trump has also advocating for streamlining the legal U.S. immigration system.

There was little mention of that in his latest policy proposal. Instead, it relied heavily on plans designed to protect American jobs from foreign workers and called for tighter rules on visas for high-skilled workers. In crafting his plan, Trump sought advice from Republican Sen. Jeff Sessions of Alabama, a longtime supporter of curbing both legal and illegal immigration.

“The influx of foreign workers holds down salaries, keeps unemployment high, and makes it difficult for poor and working class Americans — including immigrants themselves and their children — to earn a middle class wage,” Trump wrote in his proposal.

Trump’s first policy paper comes somewhat begrudgingly. In a press conference Saturday, he downplayed voters’ interest in such policy specifics, calling them a preoccupation of the press.

“I think the press is more eager to see it than the voters, to be honest,” Trump told reporters in Iowa Sunday. “I don’t think the people care. I think they trust me. I think they know I’m going to make good deals for them.”