WASHINGTON - With his unilateral decision to reprieve hundreds of thousands of young illegal immigrants from the threat of deportation, President Obama is flexing some executive muscle that some fear is overly juiced.

In effect, Obama is doing by himself what Congress so far has spurned. He says the law is on his side. Others aren't so sure. At the very least, the Democratic president, who once criticized his Republican predecessor as overreaching, once more is pressing his own aggressive interpretation of executive branch power.

"Congress," argued Sen. Charles Grassley of Iowa, the ranking Republican member of the Senate Judiciary Committee, "has the authority to write immigration laws."

Technically speaking, the fingerprints on the deportation-pause policy announced Friday belong to Homeland Security Secretary Janet Napolitano rather than Obama. Though Obama announced the decision, he was not issuing an executive order. Rather, the policy was itemized in a three-page memo issued by Napolitano.

Citing the department's "prosecutorial discretion," the memo specifies the population of illegal immigrants that will essentially be protected from deportation: those younger than 30, those who were younger than 16 when they entered the United States and who are either students, high school graduates or veterans.

The protected population is similar to that covered by the so-called DREAM Act, legislation that has failed to pass Congress since it was introduced in August 2001. Obama's executive branch actions, though, do not go as far as the legislation.

"It is not a right to stay indefinitely; it would not grant benefits generally associated with visas [and] it would not allow students to enter or leave the country with authorization," said Raquel Aldana, a professor at the University of the Pacific McGeorge School of Law. "It could go away the moment a new administration comes in or the moment this administration decides to end it."

The executive branch's authority to defer deportation, as an act of prosecutorial discretion, is not explicitly spelled out either in regulations or in statutory law. The Immigration and Nationality Act, though, gives the Department of Homeland Security the general authority to enforce immigration laws. Federal courts, moreover, have recognized that officials can exercise their discretion in determining deportation priorities.

This prosecutorial discretion is sometimes called "deferred action." Aldana said that this could be the largest-ever application of deferred action, if the estimates are correct that upward of 800,000 illegal immigrants may benefit.

Scholarly skeptics of the president's action can question whether shielding such a large population is an appropriate use of a policy that courts have said exists for the "administrative convenience" of an agency juggling different priorities amid limited resources. Napolitano herself, in a June 2011 Senate hearing, cautioned that the policy has its limits. "Deferred action, a form of prosecutorial discretion, is not a form of relief from removal exercised on a categorical basis for large classes of aliens," she said.

Deferred action is different from other ways in which presidents have protected certain populations. Previously, presidents have granted immigration parole to Cubans and Haitian orphans while temporary protected status has kept natives of Syria and Somalia from being sent back to dangerous homelands.