Washington: The US Supreme Court weighs a major immigration case Monday that could impact the fate of millions of people facing possible deportation and further raise the stakes in the 2016 White House race.

The eight justices are due to determine the legality of an ambitious bid by President Barack Obama to shield from deportation nearly five million undocumented immigrants.

Immigration is already an explosive issue in the presidential race, where Republican frontrunner Donald Trump has sharpened fault lines by vowing to build a wall along the Mexican border, and deport all 11 million or so undocumented migrants living in the United States.

But the Supreme Court hearing also puts the spotlight on Obama’s efforts to circumvent a hostile Congress, with his detractors accusing him of unfair, even illegal, overreach.

At stake are a series of executive actions taken by Obama in November 2014, after he failed to enact a promised immigration reform with a Congress held by his Republican foes.

One initiative shields from deportation people who have lived in the country since 2010, with no criminal record, and with children who are American citizens or lawful residents. Another protects immigrants who entered the United States before they turned 16.

The US federal government under Obama has upheld the policy of deporting migrants, at a rate that rose from 390,000 the year he took power in 2009, to peak at almost 440,000 in fiscal year 2013, according to figures from the Department of Homeland Security.

In unveiling his actions, however, Obama stressed that wanted to prioritize deportations of ‘felons, not families. Criminals not children. Gang members, not a mom who’s working hard to provide for her kids.’

Twenty-six states, almost all of them Republican-led, have refused to apply the measures, securing a string of court rulings that found Obama to have overstepped his authority.

‘Balance of power’

Republicans brought the legal dispute as part of broader efforts targeting signature Obama reforms such as the sweeping health care law commonly known as Obamacare.

‘This lawsuit to stop President Obama’s illegal immigration policy is about a concept as old as the nation’s founding — the separation of powers,’ Texas Attorney General Ken Paxton said in a statement.

‘No president has the power to override the will of the people as vested in our Constitution.’

For Cornell University Law School professor Stephen Yale-Loehr, the case could have wider repercussions, by indicating how the Supreme Court views executive actions in general.

‘The Court’s decision in June could redefine the balance of power between Congress and the president,’ he said.

‘People involved in other controversies involving executive actions, such as gun control or environmental policy, will certainly review the oral arguments in United States v. Texas closely to try to determine how legal challenges in those areas might fare.’

Policy on hold

Mirroring Obama’s gridlock with Congress is that of a Supreme Court evenly split between liberals and conservatives while Senate Republicans refuse to hold hearings, let alone a vote, on replacing the ninth seat left vacant in February by the death of justice Antonin Scalia.

For now, the Obama immigration policy is on hold under lower courts’ orders, so even a Supreme Court ruling — expected by late June — in the administration’s favour would only leave about seven months to implement the program before the president leaves office in January.

The outcome of the presidential race will also largely determine the future of these policies, which Republican candidates have vowed to repeal.

‘That doesn’t mean they would take it away from the people who have received it,’ tempered Stephen Legomsky, who served as chief counsel at US Citizenship and Immigration Services under Obama.

Sufficient grounds to sue?

The justices are examining several narrow questions on the immigration case, such as whether immigration initiatives impact state governments enough to give them the legal ‘standing’ to sue the federal government.

That question is key to Texas, which claimed it would cost the state millions of dollars in public funds to provide driver’s licenses to the huge group of immigrants who would be allowed to stay in the United States.

The Supreme Court is also being asked to determine whether Obama’s 2014 actions were ‘arbitrary and capricious’ under federal law and whether they should have followed a lengthy public review process.

In addition, the justices are weighing whether Obama fulfilled his constitutional duty to ‘faithfully execute’ existing federal immigration laws passed by Congress in trying to make changes through executive action.

If the justices can’t agree, lower court rulings — which largely sided with the states against the Obama administration — stand.

But Legomsky expressed confidence the government would prevail.

‘The Supreme Court, for more than 100 years, has consistently held that immigration is exclusively a federal responsibility,’ he said. ‘No one individual state should be allowed to disrupt federal immigration policy.’