[...] as a self-imposed, end-of-summer deadline to act approaches, Obama's lawyers are carefully crafting a legal rationale they believe will withstand scrutiny and survive any court challenges, administration officials say. While Obama has yet to receive the formal recommendations he's requested from Homeland Security Secretary Jeh Johnson, administration officials said the president is intimately familiar with the universe of options and won't spend much time deliberating once Johnson delivers his report. The most sweeping, controversial step under consideration involves halting deportation for millions, a major expansion of a 2012 Obama program that deferred prosecutions for those brought here illegally as children. [...] while prosecutors are routinely expected to use their discretion on a case-by-case basis, such blanket exempting of entire categories of people has never been done 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.