In a move designed to win support for immigration reform from get-tough conservatives, President George W. Bush plans to send thousands of National Guard troops to help secure the Mexican border. Bush, in a speech to the nation Monday, will propose using the troops as a stopgap measure while the Border Patrol builds up its resources to more effectively secure the 2,000-mile (3,220-kilometer) line between the US and Mexico, said two White House officials speaking on a condition of anonymity before the president is scheduled to speak at 8 p.m. EDT (0000 GMT). In a signal of the high stakes on the issue, Bush was to make his case in a rare prime-time address from the Oval Office. He planned to follow up the address with a visit Thursday to the border in Arizona. The broadcast networks ABC, CBS, NBC and Fox and the cable networks Fox News Channel, CNN and MSNBC planned live coverage of the speech, which begins at 8 p.m. EDT (0000 GMT). Bush was to call for the troops to play a supportive role to Border Patrol agents, who would maintain primary responsibility for physically guarding the border. Bush also will mention the need for immigrants to learn English and assimilate into American culture if they are to become citizens. But lawmakers have expressed concern about overextending a National Guard force that is already tied up in Iraq. And Mexican President Vicente Fox called Bush Sunday to express concern about what he called the possibility of a "militarized" border between the two nations. On Monday, White House counselor Dan Bartlett said the move does not represent "a militarization of the borders." Appearing on CBS's "The Early Show," Bartlett said Guard forces sent to the area "will not have law enforcmeent responsibilities or powers. They will be there in a supportive role. ... It's about a constitutional responsibility to enforce our borders." White House spokeswoman Maria Tamburri had said earlier that Bush made clear to Fox that "the United States considered Mexico a friend and that what is being considered is not militarization of the border, but support of border capabilities on a temporary basis by the National Guard." Bush also assured 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. Bush hopes adding the National Guard troops to make the border more secure will persuade congressional conservatives to approve a broader immigration bill that would include his primary objective - creating temporary work permits for foreigners to enter the country and take low-paying jobs. That idea is favored by the business community, but many conservatives want a tougher approach on illegal immigrants trying to sneak into the country. The officials who discussed Bush's proposal would not say how many troops he wanted to use, except that it would be in the thousands but less than 10,000 - an estimate being discussed at the Pentagon. About 100 National Guard troops are serving on the border to assist with counter-drug operations, heavy equipment support and other functions. Some lawmakers say an increase is unwise. "We've got National Guard members on their second, third and fourth tours in Iraq," said Republican Sen. Chuck Hagel of Nebraska. "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." Hagel said the bill under debate in the Senate that he helped write would double the 12,000-strong Border Patrol force over the next five years. "That's the way to fix it, not further stretching the National Guard," he said on ABC's "This Week." Democratic Sen. Joe Biden of Delaware said there may be a need for troops to fill in while the Border Patrol is bolstered. But he did not seem confident that the National Guard could take on the extra duty. "We have stretched these men and women so thin, so thin, because of the bad mistakes done by the civilians in the military here, that I wonder how they're going to be able to do it," Biden said, also on ABC. Senate Majority Leader Bill Frist, a Tennessee Republican, defended Bush's plan. He said lawmakers who doubt that the National Guard could take on border patrol duty are "whining" and "moaning." "We've got to secure our borders," Frist said on CNN. "We hear it from the American people. We've got millions of people coming across that border. First and foremost, secure the border, whatever it takes. Everything else we've done has failed. We've got to face that. And so we need to bring in, I believe, the National Guard."