Alan Gomez

USA TODAY

President Obama will deliver a speech Thursday night outlining his long-awaited plan to change portions of the nation's immigration system.

The president says he has been forced to act because Congress has been unwilling to pass any immigration law throughout his six-year tenure. Republicans in Congress call his action an unlawful use of his presidential authority that violates the basic tenets of the Constitution.

USA TODAY looks at five questions about this historic move.

Q: What do we know about the president's plan?

A: The biggest and most controversial part of the plan is the temporary legalization of up to 5 million undocumented immigrants living in the USA.

The White House is focusing on undocumented immigrants who have children who were born in the USA, meaning they are U.S. citizens by birth. Immigration advocacy groups and many Democrats have long complained that too many of those parents have been deported, separating their families and leaving those children behind with relatives or in foster care. The president is also considering protections for people who have been in the country for long periods of time and established themselves in the community.

The plan would not grant them citizenship or legal permanent residence like a "green card." Instead, it would allow people to register with the federal government and, if they cleared a series of background checks and other requirements, they would be shielded from deportation proceedings. They would be given work permits and assigned Social Security numbers, so they could legally work and pay taxes.

That move would serve as an extension of a 2012 program Obama created that protected undocumented immigrants who were brought into the country as children. The Deferred Action for Childhood Arrivals program, or DACA, has approved more than 580,000 undocumented immigrants.

The president could expand the pool of young undocumented immigrants eligible for DACA. Only people who have lived in the USA since June 15, 2007, can apply. The president could change that cutoff date, allowing more recent arrivals to legally stay.

Q: Will the president's order tackle border security?

A: Department of Homeland Security Secretary Jeh Johnson said during a speech Wednesday that the president's action will include improvements to border security. He didn't specify what actions those would be, but they could include a wide range of changes.

Obama could approve pay raises for Border Patrol agents and Immigration and Customs Enforcement officers and could redeploy enforcement officers from the interior of the country to the southwest border. During the summer, when tens of thousands of undocumented immigrants were flooding the U.S.-Mexican border, the White House discussed plans to send more Border Patrol agents, immigration judges and U.S. attorneys to speed up deportations. Those plans haven't been finalized.

This is one of the main immigration-related concerns for Republicans, who say the president has not done enough to secure the borders.

The Senate approved a plan last year that would have authorized $38 billion to improve border security, including a surge of 20,000 more Border Patrol agents and a new fleet of technology, including drones, radars and sensors. The GOP-controlled House never voted on the bill.

Q: What other areas could Obama address?

A: The president could tweak the legal immigration system to allow more foreign workers to get into the country. Silicon Valley business leaders have pressed Congress to allow more visas for high-tech workers to meet the demand in their field, and the president has some leeway to open those doors a bit more.

Obama is also likely to make changes to an immigration enforcement tool called Secure Communities.

That program allows police agencies to check the identity of anybody they've arrested against federal immigration databases. If they find immigration violations, police can hold the suspect until an ICE agent picks them up. Some law enforcement agencies and politicians have tried to opt out of the program, arguing that using officers to check immigration crimes erodes the trust between their citizens and police. Supporters say the program is necessary to ensure federal officials know when serious immigration violators are arrested around the country.

Q: Can Obama do these things unilaterally?

A: The White House says so.

Laws require the president, through Homeland Security, to carry out immigration enforcement and deport people who have violated those immigration laws. Those same laws say Homeland Security has the discretion to decide how best to implement them.

Using that discretion, the administration has established a plan to focus its limited enforcement dollars on undocumented immigrants with serious criminal records, who pose a threat to national security or have recently crossed the border. Since Homeland Security continues deporting about 400,000 people a year, White House officials say they're fulfilling their enforcement requirements.

Republicans sharply disagree. They say the discretion afforded to the executive branch was always meant to be used one a case-by-case basis. Some, like Rep. Bob Goodlatte, R-Va., chairman of the House Judiciary Committee, say Obama has overstepped those guidelines by protecting such broad classes of people and will venture further into illegal territory by expanding those pools.

Q: How will congressional Republicans react to the president's announcement?

A: One thing is clear: it won't be pretty.

Incoming Senate Majority Leader Mitch McConnell, R-Ky., has said a unilateral move by the president would "poison the well" for the prospect of a comprehensive immigration bill making it through the next Congress. House Speaker John Boehner, R-Ohio, said the president was playing with matches and he was "going to get burned."

Or, as Rep. Mario Diaz-Balart, R-Fla., one of the strongest supporters of plans to grant citizenship to undocumented immigrants, put it: "It would be like pulling the pin off a hand grenade and tossing it into the middle of the room."

But the options to stop the president are limited.

There's impeachment. Some, like Rep. Joe Barton, R-Texas, have said they have to consider impeaching the president if he follows through. Since an impeachment requires a two-thirds vote in the Senate, the chances of that happening are small.

There are the courts. Boehner said over the summer that he was planning to sue the president in federal court on the basis that he's violating the Constitution with his executive actions addressing education, energy, health care, foreign policy and immigration.There is little precedent for such a move, and Boehner has not yet filed a complaint.

And then there's the budget. The federal government's funding expires on Dec. 11, so lawmakers will spend the week after Thanksgiving crafting either a long-term budget or another continuing resolution to keep the government funded.

Six GOP senators, including Sen. Ted Cruz, R-Texas, and 62 Republican members of the House, led by Rep. Matt Salmon, R-Ariz., have signed letters calling on congressional leaders to use that budget debate to de-fund the president's order. Formally legalizing millions of undocumented immigrants requires federal workers to spend time and money on the process, and these Republicans want to craft budget bills that expressly forbid money being used to implement the president's order.