Those concerns have been stoked by reports that Homeland Security Secretary Michael Chertoff favors a high-tech solution utilizing motion detectors and other kinds of sensors, cameras, ground-based radars and a variety of computer-driven devices to help police the border.

Critics of the high-tech approach say the experience with the double-layered fencing erected a decade ago along San Diego County's border with Tijuana shows that only a physical impediment is likely to be effective. They worry that any other kind of solution could be circumvented, possibly by smugglers who overwhelm the system's vulnerable points by unleashing vast numbers of illegal border-crossers at one time.

Asked about Chertoff's reported preference for a virtual barrier versus a physical fence, Bush said, “We're going to do both,” including, he added, utilizing a “combination of fencing and technologies – UAVs, sensors.”

Later, Rep. Duncan Hunter, R-Alpine, who co-sponsored legislation requiring the construction of double-layered fencing, said in a telephone interview: “The president will build that fence. I am confident of that. And certainly we will use surveillance, but it will not be a substitute for fencing where you can build fencing.”

Some conservatives fear that the administration may not fully fund the 700-mile fence, estimated to cost $12 billion. Bush did not address that concern. So far, only a $1.2 billion down payment has been appropriated to build the fence.

Bush also used the news conference to reiterate his support for a guest-worker program as part of what he described as a “comprehensive” solution to the highly politicized problem of an illegal-immigrant population in the United States of about 12 million.

Many Republican candidates in Senate and House races far removed from the U.S.-Mexican border have denounced such plans for providing opportunities for many illegal immigrants to become legal residents over time. At least some Democratic candidates who have embraced legislation patterned after the White House's plan have been attacked by their GOP adversaries as favoring “amnesty” for illegal immigrants.

Bush did not directly address the policy and political disconnect between his immigration policy and the preference of many Republicans for an enforcement-only approach.

He did argue: “When you've got a situation where people are sneaking in to do jobs Americans aren't doing, it's . . . going to keep a strain on the border. And so, therefore, a temporary-worker plan to me makes sense, and it's a much more humane . . . approach.”