WASHINGTON - Under pressure from fellow conservatives on immigration, President Bush was going before the nation Monday night with a prime-time speech calling for deployment of National Guard troops in states along the Mexican border.

Bush acted as the Senate began debate on sweeping immigration legislation. Democratic leader Harry Reid sought to pre-empt the Oval Office address with pointed questions for the president as well as a demand.

"He must publicly denounce" a House-passed bill that makes all illegal immigrants subject to prosecution as felons and calls for construction of a 700-mile fence along the Mexican border, Reid said on the Senate floor.

White House spokesman Tony Snow said Bush had decided to propose the use of thousands of troops to support the Border Patrol while it builds up its resources to more effectively secure the 2,000-mile line between the U.S. and Mexico.

Snow said the effort would use "a very small percentage of the Guard," which numbers about 400,000 members around the country. White House officials say it would involve fewer than 8,000 Guardsmen.

In a signal of the high stakes on the issue, Bush arranged to make the announcement at 8 p.m. EDT, in a rare prime-time speech from the Oval Office. He planned to follow up with a visit Thursday to the border, in Yuma, Ariz., to further press his case.

Reid and Senate Majority Leader Bill Frist both said they expected the Senate to pass comprehensive immigration legislation by Memorial Day. The measure before the Senate calls for greater border security, new guest worker programs and an eventual chance at citizenship for most of the estimated 11-12 million illegal immigrants currently in the country.

Bush has signaled overall approval of the measure several times, but has yet to endorse the bill, which has drawn fierce criticism from House Republicans whose support will be needed to pass an eventual compromise measure.

Reid challenged Bush to distance himself from some of the conservatives' demands.

In addition, he asked Bush to say "what kind of immigration reform he supports," as well as what he will propose on border security.

Reid posed two other questions. "If President Bush is going to get tough on border security, will he finally get tough on employer sanctions as well?" and how will the National Guard troops be deployed without "jeopardizing the critical role" they play in their regular duties.

Conservative Republican Sen. Jeff Sessions (news, bio, voting record) of Alabama said sending the National Guard to improve border security would indicate Bush "gets it" and would be helpful in allaying concerns of lawmakers who oppose the Senate legislation. The impact on House critics of the measure was unclear.

Though some Republicans defended Bush's plan, others on both sides of the aisle expressed concern about overextending a National Guard force that is already tied up in Iraq and must be at the ready for disaster relief.

"We've got National Guard members on their second, third and fourth tours in Iraq," said Sen. Chuck Hagel (news, bio, voting record), R-Neb. "We have stretched our military as thin as we have ever seen it in modern times. 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? That's not their role."

Under Bush's proposal, formed in consultation with border-state governors who command the National Guard, Border Patrol agents would maintain primary responsibility for physically guarding the border. National Guard soldiers will not perform law enforcement duties, but will help in such areas as construction, surveillance and transportation, Snow said.

Snow would not say how long it would take to beef up the Border Patrol to the point where the National Guard troops would no longer be needed — or how the operation would be funded. But he said Bush's proposal goes further in beefing up border enforcement than do competing immigrations bills in the House and Senate.

In his speech, Bush also will mention the need for immigrants to learn English and assimilate into American culture if they are to become citizens.

Mexican President Vicente Fox called Bush Sunday to express concern about what he called the possibility of a "militarized" border between the two nations.

Snow said Bush assured Fox that there is "no attempt to militarize the border." The president told Fox that any military support would be administrative and logistical and would come from the National Guard and not the Army, according to a news release from Fox's office.