President Bush will use a prime time speech Monday night to call for thousands of National Guard troops to be deployed along the Mexican border to help back up the U.S. Border Patrol.

The remarks come as the Senate tries to pass a guest worker proposal that opponents say doesn't do enough to stem the influx of illegals passing over the southern border each day. Senators begin debate anew Monday on an immigration reform package that aims to allow the estimated 11 million to 12 million illegals in the United States look for a way to stay here.

The president supports a guest worker plan, but also must answer to many in his base who say border security is step one in dealing with the flood of illegal immigrants.

As a result, Bush is also being forced to walk a diplomatic tightrope with Mexican President Vicente Fox, who called Bush on Sunday to discuss efforts to improve border security. Bush emphasized to Fox that any move to put military units at the southern U.S. border was not an intimidation tactic aimed at his government, White House spokeswoman Maria Tamburri told FOX News.

Watch President Bush's primetime address Monday night at 8 p.m. EDT on FOX News Channel.

"Mexican President Vicente Fox reached out to President Bush this morning to relay his concerns about consideration of a plan by the United States to deploy National Guard forces to the border region. President Bush made clear that the United States considers Mexico a friend and that 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," Tamburri said.

"The president reiterated to President Fox his commitment to comprehensive immigration reform," she added. Fox's office also released a statement saying Bush assured the Mexican president that any military support would be administrative and logistical and would come from the National Guard and not the Army.

White House aides were working Sunday night on the details of the president's proposal to deploy National Guardsmen along the border and how to address the concerns of critics, including Govs. Arnold Schwarzenegger of California and Bill Richardson of New Mexico, among others, who say using Guardsmen to man the border would further burden an overextended military. Domestically, governors traditionally are the ones with the authority to redeploy National Guard troops within their states.

"What in the world are we talking about here? Sending a National Guard (for whom) we may not have any capacity to send down to protect the borders? That's not their role," said Sen. Chuck Hagel, R-Neb., on ABC's "This Week."

"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," added Sen. Joe Biden, D-Del.

Hagel said he helped write a bill under debate in the Senate that 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 Sunday, the president's national security adviser, Stephen Hadley, would not say how many troops the president wanted to use, but he did say the purpose would be to support, not replace, Border Patrol.

"Safeguarding the border is for the Border Patrol. And they have a huge task. And one of the issues is: do they need help on an interim basis so that they could do the full function? It's the support function we're talking about," Hadley told CBS' "Face the Nation."

A White House official said Sunday evening that Bush will propose using troops as a temporary measure while the Border Patrol builds up its resources. The troops would play a supportive role to Border Patrol agents, who would maintain primary responsibility for physically guarding the border.

The official, speaking on a condition of anonymity ahead of the address, would not give an exact number for how many troops Bush wanted to use, except that it would be in the thousands. The official said the number will be less than an estimate of 10,000 that was 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.

"I think what it would be is simply expanding the kind of thing that has already been done in the past in order to provide a bit of a stopgap as the Border Patrol build up their capacity to deal with this challenge," Hadley said.

But Rep. Charlie Norwood, R-Ga., a border security proponent and backer of the Minuteman groups shoring up the border with volunteer "spotters," wrote in an editorial Sunday in Human Events Online that Arizona Gov. Janet Napolitano already tried to redeploy 170 National Guardsmen on the troops and it didn't do much because the numbers were too small and their role was to help Border Patrol with cargo screening, not guarding the unprotected areas along the border.

Norwood said the truth in Bush's commitment to border security "will lie in the proposed numbers, and whether the plan is for a short-term demonstration project or a long-term strategy for truly securing our southern border."

He added that research data done by the House Immigration Reform Caucus shows that "the southern border can be virtually closed except at legal points of entry within a one-month period — at the longest. ... It will initially take 36,000 troops.

"At the start, they should be National Guard personnel drawn nationally. There isn't enough National Guard in the border states alone to do the job without hindering combat readiness, so the forces will need to be pulled from other states as well under current National Guard Bureau assistance regulations," Norwood said.

That's the position favored by the Minuteman Project, whose leader Jim Gilchrist told FOX News he thinks the president's proposal will just be a "head fake."

"Apparently, they just do not get the message and 5,000 National Guardsmen is not enough. You need 25,000 Guardsmen ... to supplement the Border Patrol until they can find another 25,000 agents and train them. That will take several years," Gilchrist said.

Senators will be listening closely to the president as they negotiate an immigration bill that will likely take up to two weeks to pass. The measure in the Senate is heavy on a guest worker plan, though Senate Majority Leader Bill Frist, R-Tenn., has repeatedly called for tightening the borders.

"We've got to secure our borders," Frist said on a cable news network on Sunday. "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."

He added that any lawmakers who have doubts about the National Guard — well trained after years in Iraq and Gulf Coast duty following Hurricane Katrina last year — are "whining" and "moaning."

Bush will deliver his speech from the Oval Office, where he is expected to place equal emphasis on protecting the border and accommodating many of the millions of illegal immigrants already in this country.

Specifically, he is likely to argue that many of the illegal immigrants in the United States now should be allowed to pursue a path to citizenship that would include learning English and paying back taxes and fines, all elements of the Senate bill.

But the Senate bill is a long way from the House version of immigration reform passed last December. That measure does not address the guest worker issue but increases penalties for illegal immigration activities and funds a 700-mile border fence.

Former House Speaker Newt Gingrich said if Bush can push through an immigration reform bill that avoids amnesty but tightens the borders, it could go a long way toward improving the president's sagging poll numbers.

"I think that the president, by doing the things that resonate with the American people, could rebound very quickly. I've seen presidents come back dramatically when people say they go the message, now they're doing the things I want," Gingrich told FOX News.

One million or more supporters of the guest worker plan demonstrated in April in support of a Senate compromise. But Gilchrist said many more Americans won't stand for compromise.

"We want results. Show us the results and then you'll get our support. Until we see absolute results and the enforcement of our immigration laws, you'll get no support from tens of millions, if not hundreds of millions of Americans. What the Democrats and Republicans are doing is opening up the doorway for a third party to reside in the White House as early as 2008," Gilchrist said.

FOX News' Megyn Kendall, Julie Kirtz and Sharon Kehnemui Liss and The Associated Press contributed to this report.