Revealed: The politics behind Obama's DREAM shift

At the same time, Senator Marco Rubio of Florida, a Republican whose star is rapidly rising in his party, was close to introducing his own bill to help illegal immigrant students by giving them a temporary status, something quite similar to what White House officials had in mind. They feared Mr. Rubio’s proposal would pre-empt the president, making it appear he did not want to work with Republicans.

The White House was also awaiting a ruling from the Supreme Court, expected any day, on the administration’s lawsuit against Arizona over a tough state immigration enforcement law. Campaign officials feared an adverse decision could leave Mr. Obama empty-handed when he tried to mobilize Latino voters for the November election.

A big concern for Mr. Obama, White House officials said, was whether he had legal authority to offer relief to so many immigrants. In recent weeks, the White House counsel, Kathryn Ruemmler, and Homeland Security lawyers pored over the law and concluded they were on firm ground. The main point, the officials said, was that the policy would have to be carried out case by case — meaning the workload for the immigration bureaucracy would be huge.

The White House was less concerned about whether it would be circumventing Congress and enraging Republicans. “Look, every time we sneeze in the direction of an immigrant, someone says it’s amnesty,” the official said.