The White House is crafting a blame-it-on-Congress legal justification to back up President Barack Obama’s impending executive actions on immigration.

Facing an expected onslaught of opposition, the administration plans to argue that Congress failed to provide enough resources to fully enforce U.S. laws, thereby ceding wide latitude to White House to prioritize deportations of the 11.5 million people who are in the country illegally, administration officials and legal experts said.

But Republicans, too, are exploring their legal options for stopping Obama from what they have deemed egregious presidential overreaching.

Obama’s self-imposed, end-of-summer deadline to act on immigration is rapidly approaching. While he has yet to receive the formal recommendations he has requested from Homeland Security Secretary Jeh Johnson, administration officials said the president is intimately familiar with the options and will spend little time deliberating once he receives the recommendations.

After resisting calls to act alone in hopes Congress would pass a comprehensive immigration fix, Obama in June bowed to immigration activists and said that “if Congress will not do their job, at least we can do ours.” The most sweeping step under consideration involves halting deportation for millions, a major expansion of a 2012 Obama program that deferred prosecutions for those brought to the U.S. illegally as children.

Roughly half a million immigrants have benefited from that program, known as Deferred Action for Childhood Arrivals or DACA.

But while prosecutors are routinely expected to use their discretion on a case-by-case basis, there has never been a blanket exemption on the scale of what Obama is considering — potentially involving many millions of people if he extends relief to parents of DACA children, close relatives of U.S. citizens or immigrants with clean criminal records.

“The question is how broadly can the president extend the categories and still stay on the side of spectrum of ensuring the laws are faithfully executed?” said Cristina Rodriguez, who left the Justice Department’s Office of Legal Counsel in 2013 to teach at Yale Law School.

Other options under consideration, such as changes to how green cards are distributed and counted, might be less controversial because of support from the business community and other influential groups. But Derrick Morgan, who was an adviser to former Vice President Dick Cheney and is a scholar at the conservative Heritage Foundation, said Obama will still face staunch opposition as long as he attempts an end run around Congress.

Obama’s goal had been to announce his decision around Labor Day before leaving on a trip to Estonia and Wales. But a host of national security crises have pushed the announcement back, likely until after Obama returns, said the administraton officials, who were not authorized to comment by name and demanded anonymity.

Obama’s actions will almost surely be challenged in court. What’s more, Obama may have undermined his case because he has has insisted time and again that he is the president, not the king, and “can’t just make the laws up by myself.” In a 2012 interview with Telemundo, Obama defended his decision to defer deportations for children but said he could go no farther.

“If we start broadening that, then essentially I would be ignoring the law in a way that I think would be very difficult to defend legally. So that’s not an option,” he said.

Republicans are already hinting that they will consider legal action to thwart what they have denounced as a violation of the separation of powers. House Speaker John Boehner, R-Ohio, in a conference call this month with GOP House members, accused Obama of “threatening to rewrite our immigration laws unilaterally.”

The House already has passed legislation to block Obama from expanding DACA and, through its power of the purse, could attempt to cut off the funds that would be needed to implement the expansion.