President Bush will make a broad appeal for immigration reform when he speaks to the nation tonight, but President Fox of Mexico and a key Republican senator from Mr. Bush’s home state have already criticized the policies he is set to unveil.

An alarmed Mr. Fox called Mr. Bush yesterday morning to express his concern at White House press briefings that claim the president will authorize tonight the deployment of as many as 10,000 National Guard troops along the American border with Mexico.

Mr. Bush assured his opposite number that “the United States considered Mexico a friend and that [the deployment] being considered is not militarization of the border, but support of border capabilities on a temporary basis by the National Guard,” a White House spokeswoman, Maria Tamburri, told the AP.

A military spokeswoman, Cynthia Smith, confirmed the White House position that any new military “support for Customs and Border Protection operations would be temporary in nature and allow CBP to recruit and train additional personnel.”

But some of Mr. Bush’s Republican allies voiced opposition even to a temporary military deployment.

“I don’t believe that’s the best use of our resources,” Senator Cornyn, a Republican of Texas, said. “I think obviously we need more Border Patrol agents, who are law enforcement officers, as opposed to the military. We need a lot more.”

“We currently have about 11,000 Border Patrol agents on our southern border,” Mr. Cornyn said at a press conference late last week. “You look at, for example, the number of police officers in New York City. It’s about 40,000 police officers. So we really haven’t tried to provide enough boots on the ground in order to provide that security.”

Mr. Cornyn also suggested deploying advanced ground sensors, cameras, and unmanned aerial vehicles similar to those used in American counterterrorism operations to help patrol the border.

Senator Hagel, a Republican of Nebraska, who supports the compromise bill now under consideration in the Senate, told ABC’s “This Week” that border protection was “not the role of our National Guard.”

Senator Reid, a Democrat of Nevada, agreed. Mr. Reid said Friday that National Guardsmen, who are already heavily committed to American operations abroad, should not have to endure further service as border patrollers.

Mr. Reid supports a bill that incorporates border security, temporary work permits, and an earned citizenship program for illegal immigrants.

While Mr. Bush is likely to cast the immigration debate in terms on which Republicans agree – national security and law enforcement – significant obstacles remain before the bill can clear the Senate, including the nature of the amendments debated and voted on.

Mr. Cornyn said Thursday he did not support the Senate bill in its present form, but looked forward to offering amendments to it, and reiterated his commitment to passing a comprehensive immigration bill.

“My hope is that we’ll be successful getting a bill to the floor … and out to the president’s desk, that will provide for needed border security, interior enforcement, worksite verification, and sanctions for those who continue to hire people outside of the legal system, but also provide for a temporary worker program, and a second chance for people who are currently here living in the shadows – without amnesty.”

Some observers doubt that the Senate bill will make it through the House, where negotiations will be led by Rep. James Sensenbrenner, a Republican of Wisconsin, author of a December 2005 bill that would make every immigrant who entered America illegally guilty of a felony.

The plight of the bill in the House could also be worsened by Rep. Tom Tancredo, a Republican of Colorado, leader of the 92-member Immigration Reform Caucus, which conceives of immigration as a national security and law enforcement matter rather than an economic and social one.

Others were optimistic that the House would pass the bill.

“I think the prospects are probably pretty decent,” a Senate staffer who was not authorized to speak on the record and asked not to be named said. “There needs to be some deal made, and this will contain probably some security for the House.” House procedures also raise the odds of an eventual deal there. “There’s no filibuster in the House … It obviously depends on the precise rules, but for the most part, you need a majority plus one,” the staffer said. “The fact that [Rep.] Tancredo may hate it, and the fact that a lot of other people may hate it, isn’t going to necessarily kill the bill. I think in the end, you’ll be able to work something out.”