Updated

Eight Republican senators and an independent group that supports tighter limits on immigration are warning that the Obama administration is drafting a plan to "unilaterally" issue blanket amnesty for millions of illegal immigrants as it struggles to win support in Congress for an overhaul of immigration laws.

The senators who wrote the White House on Monday say they are concerned that the administration is readying a "Plan B" in case a comprehensive reform bill cannot win enough support to clear Congress.

"It seems more real than just bullying (Republicans) into a bill -- that it's a plan that they can actually put forward ... circumventing Congress," an aide told FoxNews.com on Wednesday.

In their letter, the senators -- Chuck Grassley, R-Iowa; Orrin Hatch, R-Utah; David Vitter, R-La.; Jim Bunning, R-Ky.; Saxby Chambliss, Ga.; Johnny Isakson, R-Ga.; James Inhofe, R-Okla.; and Thad Cochran, R-Miss. -- urge the president to "abandon" what they say is a move to "unilaterally extend either deferred action or parole to millions of illegal aliens in the United States."

"Such a move would further erode the American public's confidence in the federal government and its commitment to securing the borders and enforcing the laws already on the books," they wrote.

Deferred action and parole, which give illegal immigrants the ability to seek a work permit and temporary legal status, are normally granted on a case-by-case basis. But the aide said the lawmakers have learned from "sources" that the administration is considering flexing its authority to grant the status on a mass basis.

Numbers USA, an organization that presses for lower immigration levels along with humanitarian treatment of illegal immigrants, has started a petition to the president expressing "outrage" at the alleged plan.

Rosemary Jenks, director of government relations with Numbers USA, said she's been hearing for weeks from "sources close to the Democratic leadership" in both chambers that administration officials are discussing whether the Department of Homeland Security could direct staff to grant "amnesty" for all illegal immigrants in the country.

"They're trying to figure out ways around a vote," she said.

"Any attempt to force an amnesty on the American people using this underhanded method smacks of despotism," reads the fax the group is urging supporters to sign.

The White House has not responded to a request for comment.

The Department of Homeland Security estimated last year that 10.8 million undocumented residents live in the United States; other estimates have ranged higher. Any move to grant blanket legal status, even temporary, would raise questions about how Homeland Security would be able to handle the caseload. Jenks said Congress certainly wouldn't grant the administration the funding for more caseworkers.

The purported discussions of a blanket amnesty come in the middle of several concurrent and heated debates over illegal immigration. The recently signed immigration law in Arizona has divided the country, with some states trying to replicate the state's tough legislation and other jurisdictions boycotting the state in protest. The Obama administration plans to file a court challenge.

Democrats, meanwhile, have been trying to round up support for an overhaul bill in Congress, and the Interior Department is facing renewed criticism from Republican lawmakers over restrictions it places on Border Patrol officers policing the border on federal lands. Sen. Jon Kyl, R-Ariz., shocked several Arizona residents last week when he told them that Obama had said he would not beef up border security because it would leave Republicans without an incentive to pass broader immigration reforms.

Jenks said the talks about Homeland Security allowing illegal immigrants to stay are "serious."

Under the law, immigration officials can grant deferred action to temporarily postpone removing an illegal immigrant from the country. That status does not offer a guarantee that they won't face deportation, but Jenks said illegal immigrants granted parole are often allowed to seek permanent legal status.

If a "Plan B" is being discussed, it's unclear how far along the talks might be. Another GOP Senate aide said the discussions started after Sens. Dick Durbin, D-Ill., and Richard Lugar, R-Ind., called on Homeland Security Secretary Janet Napolitano in April to stop deportations of undocumented students who could earn legal status under a bill they introduced.

A Senate Democratic aide said the Obama administration never responded to the April letter.