With top conservatives crying "sellout," the president used his weekly radio address to emphasize hurdles rather than opportunities an estimated 12 million illegal immigrants will have to face if they choose to legalize their status.

"This legislation will end chain migration by limiting the relatives who can automatically receive green cards to spouses and minor children," Bush said.

He added that future immigration decisions will be based on the level of applicants' professional skills, education and English proficiency rather than family ties to those already in the United States.

Some conservative scholars have expressed concern the reform may pave the way to the United States for as many as 50 million newcomers over the next several decades because legalization will make the 12 million illegals now in the country eligible to bring in family members.

The bipartisan compromise, announced by a group of US senators with great fanfare on Thursday, establishes a temporary worker program that illegal aliens would be able to join by applying to a renewable "Z" visa and paying a 5,000-dollar fine.

But it will allow program participants to eventually seek permanent residency and citizenship, although only after returning to their countries of origin.

Bush quickly endorsed the proposal and reiterated his support Saturday, saying the reform will "restore respect for the law, and meet the legitimate needs of our economy."

But he is facing an open revolt on the Republican Party's right flank that has staunchly supported him on Iraq and now feels betrayed by the White House.

"It's a big government fantasy with no hope of becoming reality," said Newt Gingrich, a former Republican speaker of the House of Representatives who is now mulling a 2008 White House run, commenting Friday on the reform proposal.

Republican Representative Tom Tancredo called the immigration plan "a slap in the face" of hard-working Americans.

"The president is so desperate for a legacy and a domestic policy win that he is willing to sell out the American people and our national security," Tancredo said.

Republican presidential candidate Mitt Romney argued the plan amounted to amnesty while Senator Elizabeth Dole vowed to oppose it "unless it is radically altered."

Senator Jeff Sessions, an Alabama Republican who has strongly supported the president on Iraq, declared himself "deeply concerned" by the bill, while Texan John Cornyn said he "simply cannot, and will not, support any legislation" that would offer undocumented aliens amnesty.

Even Senate Republican Minority Leader Mitch McConnell, Bush's unwavering right-hand man on Capital Hill, was unable to endorse the bill, promising only to "review" it.

With the fate of the reform uncertain and relations with allies strained, Bush made it a point to emphasize elements of the bill conservatives might like.

He said illegal immigrants will be able to qualify for a temporary worker program only if they pass a strict criminal background check, pay a fine, demonstrate their ability to hold a job and learn to speak English.

"Temporary workers will be allowed to bring immediate family members only if they demonstrate that they can support them financially, and that their family members are covered by health insurance," Bush promised.

He also moved to give new hope to conservatives, who have long called for making English the official language of the United States.

"This bill affirms that English is the language of the United States," the president said without elaborating.