W ASHINGTON -- A vagrant thought crossed my mind earlier this week when interviewing Homeland Security Secretary Michael Chertoff on the president's proposal to use National Guard troops to support the Border Patrol efforts to curb illegal immigration from Mexico.

Chertoff was explaining that the rotating contingents of 6,000 troops who would supplement the 12,000 patrol agents on the border would be an interim force. It might take two years, he said, to recruit and train several thousand additional men and women for the border agency that is now part of his department. As they come on line, he said, the number of National Guard troops would be reduced.

"Sort of like Iraq," I said. "As they stand up, we will stand down."

"I would not make that comparison," Chertoff replied.

Of course he would not, because it implies that this latest Bush military deployment could be as open-ended and frustrating in its results as the previous one.

But, whatever the long-term outcome for the border, this major escalation is a significant step.

It may well be the minimum price to be paid for breaking the resistance of House members of both parties to the kind of comprehensive immigration reform the president forcefully advocated in his Monday night television address from the Oval Office.

But, as Republican Majority Leader John Boehner made clear on Tuesday morning, initially at least, it was No Sale for the president.

He said, "I understand the president's position" that tightened border security should be linked to a guest-worker program giving immigrants a way to work temporarily and legally in this country, and to a path for eventual citizenship for the millions who have been residing here illegally for many years.

"But," Boehner added, "I've made it clear that I support the House position," that tough measures should be taken now to close the border and deport the illegals, and only after that has been done should other steps be considered.

So the president's proposal faces a triple hazard in the real world. One potential pitfall: If the goal is to "seal" the border, will an increment of 6,000 National Guard troops, plus a load of fancy electronic surveillance equipment, be sufficient to do the job?

Chances are, the answer is no. According to Chertoff and others in the administration, the size of the Border Patrol has grown by 3,000 -- from 9,000 to 12,000 -- in recent years and spending on border security has gone up at an even faster pace. But the tide of illegals drawn by the promise of jobs they cannot find in their home countries still floods into the United States.

The second question is even more basic: Is the assumption that it's possible to "seal" the border at all realistic? This is where the Iraq parallel becomes even more pertinent. We invaded that country on what turned out to be a false premise -- that Saddam Hussein had a threatening cache of weapons of mass destruction. The folly of that assumption has shadowed the whole war.

There are students of the border, far more expert than I am, who say that idea of a barrier -- physical, electronic or human -- along those miles of desert is unrealistic. Their cautions deserve attention.

Finally, there is the political question: Can the House be moved by the proposal the president has put forward? The odds are against it. A forceful intervention by Bush last December, when the House bill was being drafted, might have made it less onerous. He might have failed even then, but now the chances of getting the House to move to anything like the Bush position are far worse.

Boehner would say only that if the Senate passes a bill -- as it is expected to do on terms that Bush would find acceptable -- "we will go to conference and see what can be worked out."

But he expressed his support for House Judiciary Committee Chairman James Sensenbrenner, who wrote the hard-line House legislation, as the negotiator of any deal, and asserted his belief that public opinion supports that approach -- first clamp down on the border and only then consider other steps.

Bush's stance is honorable and generous, befitting his long personal history with this issue. But he comes to it in a weakened political position and with a dubious proposal. It will be a miracle if he prevails.

David Broder writes for the Washington Post Writers Group. His e-mail address is davidbroder@washpost.com.

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