The US Supreme Court heard oral arguments Monday on President Barack Obama's executive order shielding millions of illegal immigrants from deportation.
The case is central to US immigration policy and could fuel debate on the issue in the presidential campaign.
Hundreds of protesters were out in force in front of the Supreme Court to show support for Obama's action.
Speaking outside the Supreme Court, Texas Congressman Joaquin Castro called immigration "an issue where there's a lot of emotion on both sides."
Castro, who supported Obama's immigration move, said he expects the court to uphold the president's executive action.
"Obama has the authority to make these executive actions with respect to immigration in the same way that other presidents Republican and Democrat have done for many years," Castro said.
The only difference this time, Castro said, was "that you have a group of conservative states and their leaders disagreeing with it, and so they brought a lawsuit, but it's been done many times."
Brenda Barrios, who came to the United States 13 years ago from Guatemala, does not have legal status though her two sons are US citizens. Her son Francarlo, 10, wore a T-Shirt that said: "Don't deport my dad."
Obama's order would "change everything for us," Barrios told dpa. "We will not be afraid to bring the children to the park when we listen on the news that Immigration [and Customs Enforcement] is outside. And I will not be afraid to be arrested while my children are at home."
The Supreme Court announced in January that it would hear the case, after a lower court struck down Obama's 2014 executive order, which prevents deportation of many undocumented residents whose children were born on US soil, and grants them work permits.
Obama said the order was necessary because Congress had failed to pass immigration reforms.
Arguments in the Supreme Court focused on whether Obama's order required the approval of Congress, whether the programmes expanded by the order are legal under federal law and whether state governments have legal grounds to challenge the executive branch's enforcement of a federal law.
In November, the Fifth Circuit Court of Appeals in New Orleans upheld a trial court decision that prevented the federal government from implementing the executive order.
The trial court had found that Texas, which was among 26 states suing to block the Obama policy, had a well-founded legal argument that Obama's executive order "clearly legislated a substantive rule without complying with the procedural requirements" of US law.
Texas Attorney General Ken Paxton said there were limits to the president's authority, and Obama exceeded them when he "unilaterally sought to grant 'lawful presence' to more than 4 million unauthorized aliens who are in this country unlawfully."
"It's as if the president is defining the policy and the Congress is executing it," Justice Anthony Kennedy said, according to The New York Times. "That's just upside down."
Kennedy was known as a tie-breaker on the court when it had its full complement of nine justices. Since the February death of Justice Antonin Scalia, however, the court is operating with eight members. A tie vote would mean the earlier ruling against Obama's order would stand.
Left-leaning justices argued that a solution to the immigration issue was long overdue. "They're here whether we want them or not," Justice Sonia Sotomayor said, according to The New York Times.
The Justice Department has argued that the lower court's ruling interfered with the executive branch's proper discretion and causes "far-reaching and irreparable humanitarian impact."
White House spokesman Josh Earnest said Monday: "We continue to believe in the strong legal arguments."
Immigration advocates have expressed hope that the court will rule in favour of the administration and the immigrants who stand to benefit from the policy.
A decision is expected in June.