Spurning furious Republicans, President Barack Obama unveiled expansive executive actions on immigration Thursday night to spare nearly 5 million people in the U.S. illegally from deportation and refocus enforcement efforts on “felons, not families.”

The moves, affecting mostly parents and young people, marked the most sweeping changes to the nation’s fractured immigration laws in nearly three decades and set off a fierce fight with Republicans over the limits of presidential powers.

In a televised address to the nation, Obama defended the legality of his actions and challenged GOP lawmakers to focus their energy not on blocking his actions, but on approving long-stalled legislation to take its place.

“To those members of Congress who question my authority to make our immigration system work better, or question the wisdom of me acting where Congress has failed, I have one answer: Pass a bill,” Obama said, flexing his presidential powers just two weeks after his political standing was challenged in the midterm elections.

As Obama addressed the nation from the White House, immigration supporters with American flags draped over their shoulders marched on the street outside carrying signs that read, “Gracias, Presidente Obama.”

Despite Obama’s challenge to Republicans to pass a broader immigration bill, his actions and the angry GOP response could largely stamp out prospects for Congress passing comprehensive legislation under the current administration, ensuring that the contentious debate will carry on into the 2016 presidential campaign.

Republicans, emboldened by their sweeping victories in the midterms, are weighing responses to the president’s actions that include lawsuits, a government shutdown, and in rare instances, even impeachment.

House Speaker John Boehner, R-Ohio, who has refused to have his members vote on broad immigration legislation passed by the Senate last year, said Obama’s decision to go it alone “cemented his legacy of lawlessness and squandered what little credibility he had left.”

U.S. Rep. Bill Cassidy, R-Baton Rouge, said, “President Obama is exceeding his constitutional authority, harming relationships with Congress and with the American people. The President doesn’t care that a majority of Americans, including Louisiana families, oppose illegal immigration. He also doesn’t care about offending generations of law-abiding immigrants who followed the law to earn their legal status.”

U.S. Sen. Mary Landrieu, D-La., who is facing Cassidy in a Dec. 6 runoff in her re-election bid, said, “We are all frustrated with our broken immigration system, but the way forward is not unilateral action by the president. In June of last year, the Senate passed a comprehensive immigration reform bill. The real solution to fix our immigration system is for the House of Representatives to pass or amend the bipartisan Senate bill.”

While Obama’s measures are sweeping in scope, they still leave more than half of the 11 million people living in the U.S. illegally in limbo. The president announced new deportation priorities that would compel law enforcement to focus its efforts on tracking down serious criminals and people who have recently crossed the border, while specifically placing a low priority on those who have been in the U.S. for more than 10 years.

He insisted that his actions did not amount to amnesty.

“Amnesty is the immigration system we have today — millions of people who live here without paying their taxes or playing by the rules, while politicians use the issue to scare people and whip up votes at election time,” he said.

The main beneficiaries of the president’s actions are immigrants who have been in the U.S. illegally for more than five years but whose children are citizens or lawful permanent residents. After passing background checks and paying fees, those individuals can now be granted relief from deportation for three years and get work permits. The administration expects about 4.1 million people to qualify.

Obama is also broadening his 2012 directive that deferred deportation for some young immigrants who entered the country illegally. Obama will expand eligibility to people who arrived in the U.S. as minors before 2010, instead of the current cutoff of 2007, and will lift the requirement that applicants be under 31. The expansion is expected to affect about 300,000 people.

Applications for the new deportation deferrals will begin in the spring.

Immigration-rights activists gathered at watch parties around the country to listen to the president announce actions they have sought for years.

“We’re going to have plenty of Kleenex around,” said Jorge-Mario Cabrera, spokesman for the Coalition for Humane Immigrant Rights of Los Angeles.

The White House insists Obama has the legal authority to halt deportations for parents and for people who came to the U.S. as children, primarily on humanitarian grounds. Officials also cited precedents set by previous immigration executive actions by Democratic and Republican presidents dating back to Dwight Eisenhower.