By James Oliphant

WASHINGTON (Reuters) - The U.S. Supreme Court challenge to President Barack Obama's immigration policies could have an impact far beyond determining whether millions of undocumented immigrants can remain in the country. The case has the potential to constrain the power of Obama's successor to bypass Congress and act alone.

Should Obama’s order blocking deportations for certain immigrants be invalidated by the justices, the decision could hamper future presidents' ability to craft policy through executive fiat, legal experts told Reuters.

"The question is not the merits of the immigration issue,” said T. Gerald Treece, a constitutional law professor at the South Texas College of Law in Houston. “The question is what the president's power is.”

The high court said Tuesday it would hear the lawsuit brought by 26 states that seeks to overturn Obama's 2014 executive order that shields more than 4 million immigrants in the country illegally from deportation proceedings.

The Democratic Obama White House, vexed by a hostile, Republican-controlled Congress, has employed the president's executive authority with increasing frequency.

In addition to the executive order on deportations, Obama has acted alone to alter provisions of the Affordable Care Act, limit carbon emissions to combat climate change and toughen the requirements on firearms merchants.

Should Obama lose before the Supreme Court, the case could tie the hands of a future president to act in similar ways.

The immigration case likely will be argued before the Supreme Court in April, with a decision handed down at the end of June, guaranteeing that presidential power will be a front-burner issue as the race for the White House intensifies.

In taking the case, the justices indicated they will consider whether Obama violated not just federal immigration statutes but the Constitution as well, raising the possibility that the court could articulate a forward-looking principle that limits the reach of a president’s executive authority - particularly with regard to domestic issues. Presidents historically enjoy more freedom to act unilaterally when it comes to foreign affairs.

“If the Supreme Court rules against the administration on that ground, that would have a more positive impact on the limits of the president’s power on domestic policy,” said Todd Gaziano, a constitutional law expert with the conservative Pacific Legal Foundation and a former Justice Department lawyer.

The justices’ decision to allow a constitutional challenge to Obama’s actions was seized upon by Senator Ted Cruz of Texas, a leading contender for the Republican presidential nomination.

Cruz, the former top advocate for the state of Texas before the Supreme Court, published an article last year in a Harvard Law School journal condemning what he termed the administration’s “lawlessness.”

On the stump, however, Cruz has promised to roll back the Affordable Care Act as well as unilaterally terminate the Obama administration’s nuclear pact with Iran, both examples of aggressive presidential action.

Presidents tend to favor a generous reading of their authority and resist any court-imposed limitations, said Kenneth Mayer, an executive power scholar at the University of Wisconsin.

Indeed, in 2008, Obama ran for president criticizing President George W. Bush’s expansive use of executive power, which included a warrantless wiretapping program and indefinite detention of terror suspects. Once in office, Obama continued many of Bush’s counterterrorism policies and has zealously guarded presidential power.

During his second term, Obama declared a “Year of Action” and vowed to use his “pen and phone” to issue policy directives in the face of congressional inaction. The immigration order came soon thereafter.

Cruz could experience a similar conversion should he reach the White House. “The world looks very different from the Oval Office than it does from the campaign trail,” Mayer said.

Mayer predicts the court will not go as far as to set limits on a president’s executive power, and will instead likely narrowly tailor its ruling to the immigration issue in question. Still, he concedes with conservatives holding a 5-4 majority “it’s possible the court will impose some constraints.”

Additional reporting by Jim Forsyth.

To Learn More:

Ronald Reagan’s Executive Order that Opened the Door for Spying on Americans (by Steve Straehley, AllGov)

Obama to Sign Executive Order Barring Anti-Gay Discrimination by Federal Contractors (by Noel Brinkerhoff, AllGov)

Obama Has Authoritarian Powers Bush Could Only Dream Of (by Matt Bewig, AllGov)