WASHINGTON — President Obama’s announcement of sweeping changes to the nation’s immigration system is likely to lead to a battle over the legality of his actions. Is he on solid legal ground?

For months the White House and Obama’s supporters have insisted that he has the authority to direct immigration authorities to exercise discretion in deciding which immigrants in the country illegally will face deportation and which won’t.

“The actions I’m taking are not only lawful, they’re the kinds of actions taken by every single Republican president and every Democratic president for the past half century,” Obama said Thursday night.

But Republicans in Congress disagree. They’re calling Obama’s plan an unconstitutional power grab.

“The president seems intent on provoking a constitutional crisis by adopting policies that he previously said were illegal,” Sen. John Cornyn, R-Texas, said.

Incoming Senate Majority Leader Mitch McConnell, R-Ky., said Congress will act to stop the president’s executive actions when his party takes control of the Senate in January.

Among those being protected from deportation under Obama’s plan are the parents of U.S. citizens and legal permanent residents, roughly 4.1 million people.

A senior administration official said Thursday that the decision to protect this group is in line with existing law that allows adult citizens to sponsor their parents for immigration. Obama’s plan goes a step further because the sponsoring citizen doesn’t have to be an adult.

A year after Ronald Reagan and Congress enacted an overhaul that gave legal status to up to 3 million immigrants who had no authorization to be in the country in 1986, Reagan’s Immigration and Naturalization Service expanded the program to cover minor children of parents granted amnesty. Spouses and children of couples in which one parent qualified for amnesty remained subject to deportation, leading to efforts to amend the 1986 law.

President George H.W. Bush in 1990 established a “family fairness” program in which family members who were living with a legalizing immigrant and who had been in the U.S. before passage of the 1986 law were granted protection from deportation and authorized to seek employment.

At issue is how far Obama can go on his own to shield from deportation immigrants who are in the country illegally. The administration and its supporters have argued that the use of prosecutorial discretion allows the president to decide which groups of immigrants should be a priority. Obama has argued that he can go one step further and use a provision in immigration law called “deferred action” to protect particular immigrants from deportation, and also make them eligible for work permits.

What the president can’t do is halt all deportations, or permanently change the immigration status of any specific group of immigrants. Only Congress has that power.

While Obama’s proposals have been cleared by lawyers from both the Homeland Security and Justice departments, some Republican governors meeting in Florida this week said they were weighing a lawsuit to block the president’s action.

Outgoing Texas Gov. Rick Perry said a lawsuit was “very likely,” and Indiana Gov. Mike Pence and Louisiana Gov. Bobby Jindal suggested they’d be willing to join the legal challenge.

“It should be immediately challenged in court,” Pence said.