There are some limits under federal law, and anything the White House ultimately decides to do may be challenged in court as unconstitutional. But leading legal experts say the White House almost certainly could delay indefinitely efforts to deport millions of immigrants already in the U.S. illegally, and it could give them official work permits that would allow them to legally find jobs, obtain driver’s licenses and pay income taxes.

Here is what Obama could not do without approval from Congress: He couldn’t generally give large groups of immigrants permission to remain permanently in the United States, and he couldn’t grant them American citizenship. And he couldn’t generally make them eligible for federal or state social benefit programs, such as welfare payments, food stamps or the administration’s health care plans.

“There is prosecutorial discretion which can be exercised in these sorts of situations,” said Leon Rodriguez, a former Justice Department lawyer and the newly confirmed director for U.S. Citizenship and Immigration Services. “In most enforcement realms, generally there is pretty broad discretion.” Rodriguez spoke earlier this week on Capitol Hill during an oversight hearing for the House Judiciary Committee.