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“I don’t know of any plans. I think we have discussed, as we always do, the tools available to us and whether the deployment of any of those tools could achieve a more fair and efficient use or application of the immigration law,” he said.

He acknowledged, though, that he was not aware that those powers had ever been used before on a categorical basis.

Sen. John Cornyn, the Texas Republican who queried Mr. Mayorkas on the subject, warned him against pursuing that strategy.

“I think it would be a mistake for the administration to use administrative action, like deferred action on a categorical basis, to deal with a large number of people who are here without proper legal documents to regularize their status without Congress‘ participation. I will just say that to you for what it’s worth,” Mr. Cornyn, the ranking Republican on the Senate Judiciary immigration, border security and citizenship subcommittee, told Mr. Mayorkas.

“The American public’s confidence in the federal government’s ability and commitment to enforce our immigration laws is at an all-time low,” Mr. Cornyn said in a statement. “This apparent step to circumvent Congress – and avoid a transparent debate on how to fix our broken immigration system –  threatens to further erode public confidence in its government and makes it less likely we will ever reach consensus and pass credible border security and immigration reform.”

After reports earlier this year that the agency was working on these sorts of plans, Senate Republicans, led by Mr. Grassley, have sent letters to President Obama and Homeland Security Secretary Janet Napolitano asking for details.

In one letter, the senators warned the president against making an end-run around congressional authority to write immigration rules, and asked for Mr. Obama to promise that he would not use the rules to grant mass pardons.

Rosemary Jenks, government relations manager for NumbersUSA, an organization that advocates for stricter immigration limits, said the memo is “an outrageous usurpation of congressional authority. It is unconstitutional, and a slap in the face to the American people.”

She said that the memo could explain why the push for an immigration bill has faltered in Congress.

“This makes sense of the fact that [Senate Majority Leader Harry] Reid and [House Speaker Nancy] Pelosi and Obama are sitting back calmly content with not moving immigration reform this year - because they know Obama is trying to take care of it for them, without Democrats having to be tied down to a vote before the election,” she said.

On the other side of the political spectrum, immigrant rights groups have demanded that Mr. Obama halt deportations until he secures a broad legalization bill from Congress - legislation that supporters call “comprehensive immigration reform” because it would tackle enforcement, some aspects of legal immigration and the status of illegal immigrants at the same time.

Two senators earlier this year wrote asking the administration to use its powers to stop deporting students who might be eligible for the DREAM Act, which would allow illegal immigrant college students brought to the U.S. at a young age to gain legal status. The legislation has not been passed by Congress.

Mr. Obama has rejected halting deportations, but his administration has been more careful about whom it pursues.

According to new figures from Immigration and Customs Enforcement, the administration has stepped up its efforts to deport illegal immigrants convicted of crimes, but removal of “non-criminal” illegal immigrants has slowed so far in fiscal 2010.