WASHINGTON, May 14 — President Bush's plan to send National Guard troops to patrol the southern border of the United States has raised the concern of his longtime ally President Vicente Fox of Mexico, who called Mr. Bush on Sunday to express his worries.

White House officials said Mr. Bush assured Mr. Fox that a permanent National Guard presence on the border was not being considered.

"The president made clear that the United States considers Mexico a friend," said Maria Tamburri, a White House spokeswoman.

Ms. Tamburri said the president told Mr. Fox, "What is being considered is not a militarization of the border, but support of border patrol capabilities, on a temporary basis, by National Guard personnel."

In a televised address scheduled for 8 p.m. Monday, Mr. Bush is expected to call for a significantly increased National Guard presence at the border. Officials have indicated that Mr. Bush could call for a force of thousands but that it would not be as high as 10,000, a number that had been rumored late last week.

Reports of the plan over the weekend also caused concern among lawmakers, including some Republicans, who said they feared the National Guard was already overextended with military missions abroad and with its response to natural disasters at home.

On Monday, Mr. Bush is also expected to outline several other proposals aimed at sealing the border and cracking down on workers who are illegally in the United States, and the employers who hire them. Aides said he would renew his calls for an overhaul of the nation's immigration law that includes provisions to grant illegal immigrants the right to work here legally.

The president's speech, his first on domestic policy from the Oval Office, is to come as the Senate begins trying again to pass a bill that addresses competing demands to stem the flow of workers across the border from Mexico and the desire of American employers to have reliable access to a low-wage work force.

White House officials have made it clear that they hope that a plan to seal the border will help Mr. Bush in that effort to strike a compromise between any bill passed in the Senate and the one passed in December in the House, where many Republicans have opposed any steps to legalize illegal workers.

Lawmakers from both parties expressed concerns on Sunday about the idea of deploying the National Guard.

Senator Chuck Hagel, Republican of Nebraska, who helped draft the Senate immigration bill, said he was skeptical about whether the plan would work.

"We have stretched our military as thin as we have ever seen it in modern times," Hagel said on "This Week" on ABC. "And what in the world are we talking about here, sending a National Guard that we may not have any capacity to send, up to or down to protect borders?"

He said he did not believe border protection was "the role of our National Guard."

Speaking on "Late Edition" on CNN, Senator Patrick J. Leahy, Democrat of Vermont, said expressed similar feelings, saying, , "We're stretching them pretty thin now. We're going to make a border patrol out of them?"

But White House officials said late last week that they believed the president's address on Monday would be welcomed by voters, who have told pollsters they would like to see tighter control of the borders.

"The president is looking to do everything he can to secure the border," said Stephen J. Hadley, the national security adviser, on "Face the Nation" on CBS. "It's what the American people want, it's what he wants to do."

Mr. Hadley said sending National Guard troops to the border — officials say there are about 200 there now — would supplement the Border Patrol as it adds agents whose training and deployment will take time.

White House officials said that was the message that the president conveyed to Mr. Fox, whose defense minister met on Friday with Defense Secretary Donald H. Rumsfeld at the Pentagon. Ms. Tamburri, the White House spokeswoman, said Mr. Fox and Mr. Bush also discussed "cooperative efforts under way" on the border.

A statement from Mr. Fox's office said that during the president's 30-minute conversation he reiterated to Mr. Bush his conviction that the best way to manage the problem of illegal migration was with comprehensive legislation.

Migration has been the centerpiece of Mr. Fox's foreign policy in the six years of his presidency. After the Sept. 11 terrorist attacks, his hopes faltered for swift passage of measures to legalize an estimated six million Mexicans working illegally in the United States.

The relationship between Mexico and the United States grew tense as the Bush administration began focusing more on ways to seal the border than to expand opportunities for the legal flow of migrant workers. Still, Mr. Fox publicly supported most of Mr. Bush's law enforcement efforts on the border. His cooperation with the United States has cost him significant political clout, however, among an increasing number of left-leaning leaders across Latin America.

And with presidential elections less than two months away, feelings that Mr. Fox has subordinated Mexico's sovereignty to American interests threaten to affect the chances of the candidate he hopes to succeed him, Felipe Calderón.

Mr. Fox's expression of concern to Mr. Bush, along with that of members of Congress and some governors, underscored the constituencies the president is juggling as heseeks a legislative victory on an issue of special interest to him at a time when much of his agenda is stalled.

His push for granting illegal immigrants legal status, and his veiled discussion of a path to citizenship — he often says those who want to become citizens would have to go to "the back of the line" — has been dismissed as "amnesty" by some conservatives. And, as his party faces a rough midterm election fight, Republicans have worried that his push on immigration has helped demoralize core conservative voters.

Annotazioni − Ginger Thompson contributed reporting from Mexico City for this article.