After years of pressure from immigration activists, President Barack Obama’s executive action has finally arrived. In a prime time address Thursday night, he will announce that he will use his legal authority to allow more than four million undocumented immigrants to stay and work in the United States.

"Everybody agrees that our immigration system is broken. Unfortunately, Washington has allowed the problem to fester for too long," Obama said in a Facebook video released Wednesday. "So what I'm going to be laying out is the things I can do with my lawful authority as president to make the system work better even as I continue to work with Congress and encourage them to get a bipartisan, comprehensive bill that can solve the entire problem."

The move builds upon Obama’s 2012 executive action­­—the Deferred Action for Childhood Arrivals (DACA) program—that deferred deportation of 1.2 million undocumented immigrants, so-called DREAMers, and allowed them to work. To qualify for DACA, an undocumented immigrant had to be under the age of 30 and brought to the U.S. as a child before 2007. Around 700,000 people applied for DACA and approximately 600,000 received a two-year renewable grant of deferred status.

Obama’s new action has a few components. It creates a new program for entrepreneurs to come to the U.S., eliminates the Secure Communities program, and shifts law enforcement resources to focus on criminals and those who have recently crossed the border. But the most consequential (and controversial) changes to immigration policy are modeled on his 2012 move. Nicknamed DACA 2.0, it reforms the original program by eliminating the age limit and moving the year of arrival up to 2010. The White House estimates that another 270,000 undocumented immigrants will become eligible for the program. The government will also offer deferred status to undocumented immigrants who have been in the U.S. more than five years and who are parents of American citizens or lawful permanent residents. That will make approximately four million people eligible for the program. However, it does not include the parents of DREAMers, which many immigration activists wanted. As with DACA, the new action gives work authorizations to the beneficiaries.

“Deferred action isn’t a pathway to citizenship. It’s not a legal status. It simply says for three years, because you are not an enforcement priority, we’re not going to go after you,” a senior administration official said. “While we’re busy going after terrorists and criminals, if you come forward and submit yourself to a criminal background check—assuming you meet the other eligibility requirements—we’ll allow you to work and pay taxes, because we’re not going to prosecute you for this limited period of time.”

Those work authorizations may be the most important part of the program. They allow millions of undocumented immigrants to come out of the shadows, work for a fair pay and receive protection under U.S. law. Under the original DACA program, 61 percent of beneficiaries obtained a new job. Fifty-four percent opened their first bank account and 61 percent obtained a driver’s license. Families that otherwise would have been torn apart by deportations will be kept together. For the nearly all Americans, this action will have no effect whatsoever. But for those who are affected by it, their lives will improve significantly.

“After a long and hard fought campaign, today, we celebrate our victory and those whose lives will be changed,” Cristina Jimenez, the managing director of United We Dream, said in a statement. “We also recommit to continue fighting so that millions more can experience what it’s like to live without fear of being torn from your community and family.”

The president’s authority to make this unilateral move rests on the idea of prosecutorial discretion—the federal government has limited resources to implement laws so it must prioritize them accordingly. Prosecutorial discretion, though, is not enough by itself to justify Obama’s actions. He also most ensure that he is upholding Congress’s priorities in creating immigration law, which includes deterring law-breaking and ensuring the system is transparent and fair. If he fails to do so, his actions will cross the line into lawmaking—that’s the sole duty of the legislative branch. But the president has crafted his action careful to the legal limits of his office. DACA 2.0 does not cross the line.

Obama's critics believe otherwise, arguing that he's overstepping his constitutional authority and violating political norms. In a pre-buttal to Obama’s move Wednesday, Senator Ted Cruz lashed out at the president. “It is lawless. It is unconstitutional. He is defiant and angry at the American people,” Cruz wrote in Politico. “If he acts by executive diktat, President Obama will not be acting as a president, he will be acting as a monarch.” The Texas firebrand and likely Republican presidential candidate has proposed using the government funding bill, which must be passed by December 11, to block the president’s action. That option has gained popularity among the right wing in recent weeks, potentially setting up another government shutdown fight in early 2015.

Just about every Republican—and some Democrats, like Senator Joe Donnelly—shares Cruz’s opinion, although much of the GOP leadership is hesitant to risk another shutdown after the politically toxic 2013 shutdown. The saving grace for soon-to-be Senate Majority Leader Mitch McConnell and House Speaker John Boehner is that a shutdown is not necessarily a bad political move for the Republican Party this time, as it clearly was last year. Polls show the more Americans are opposed to Obama’s action than support (though a majority supports a pathway to citizenship, with conditions). And unlike in 2013 when Democrats were unified in their opposition to defunding Obamacare, the GOP will likely receive some support from congressional Democrats. If the president vetoes a funding bill to protect the deferred status of millions undocumented immigrants, he could face a serious public backlash.

Still, it’s hard to imagine Obama curtailing or undoing this program under any circumstance. For one, the politics aren’t good: It would infuriate the Hispanic community and win him few supporters among Republicans. But Obama also believes deeply in improving the lives of the millions of undocumented immigrants in the country. The executive action, which Obama will sign in Nevada Friday, will do just that. It may go down as the greatest domestic policy achievement of his second term—and one of the greatest of his entire presidency. He’s not going to relinquish that lightly.

“The President told the GOP that he would act if they didn’t. And he is. The lives of millions of people will be improved through these actions,” Simon Rosenberg, the president of New Democrat Network, said. “They are good for our economy, for public safety and for border security. It is the president’s job to do the hard things that are good for the nation and that is what he is doing today.”