WASHINGTON -- No sooner did Congress authorize construction of a 700-mile fence on the U.S.-Mexican border last week than lawmakers rushed to approve separate legislation that ensures it will never be built, at least not as advertised, according to Republican lawmakers and immigration experts.

GOP leaders have singled out the fence as one of the primary accomplishments of the recently completed session. Many lawmakers plan to highlight their $1.2 billion down payment on its construction as they campaign in the weeks before the midterm elections.

But shortly before recessing late Friday, the House and Senate gave the Bush administration leeway to distribute the money to a combination of projects -- not just the physical barrier along the southern border. The funds may also be spent on roads, technology and "tactical infrastructure" to support the Homeland Security Department's preferred option of a "virtual fence."

What's more, in a late-night concession to win over wavering Republicans, GOP congressional leaders pledged in writing that Native American tribes, members of Congress, governors and local leaders would get a say in "the exact placement" of any structure, and that Homeland Security Secretary Michael Chertoff would have the flexibility to use alternatives "when fencing is ineffective or impractical."

The loopholes leave the Bush administration with authority to decide where, when and how long a fence will be built, except for small stretches east of San Diego and in western Arizona. Homeland Security officials have proposed a fence half as long, lawmakers said.

"It's one thing to authorize. It's another thing to actually appropriate the money and do it," said Sen. John Cornyn, R-Texas. The fine-print distinction between what Congress says it will do and what it actually pays for is a time-honored result of the checks and balances between lawmakers who oversee agencies and those who hold their purse strings.

In this case, it also reflects political calculations by GOP strategists that voters do not mind the details, and that key players -- including the administration, local leaders and the Mexican government -- oppose a fence-only approach, analysts said.

President Bush signed the $34.8 billion homeland security budget bill Wednesday in Scottsdale, Ariz., without referring to the 700-mile barrier. Instead, he highlighted the $1.2 billion that Congress provided for an unspecified blend of fencing, vehicle barriers, lighting and technology such as ground-based radar, cameras and sensors.

"That's what the people of this country want," the president said. "They want to know that we're modernizing the border so we can better secure the border."

Bush and Chertoff have said repeatedly that enforcement alone will not work and that they want limited dollars spent elsewhere, such as on a temporary-worker program to ease pressure on the border. At an estimated $3 million to $10 million per mile, the double-layered barrier will cost considerably more than $1.2 billion.

Sen. Judd Gregg, R-N.H., who chairs the Senate subcommittee that funds the Department of Homeland Security, said that before the legislation was approved, the department had planned to build 320 miles of fencing, secure 500 miles of hard-to-traverse areas by blocking roads and monitor electronically the rest of the 2,000-mile-long southern frontier.

"I think there'll be fencing where the department feels that it makes sense," Gregg said, estimating that "at least 300 to 400 miles" will be built.

Congress withheld $950 million of the $1.2 billion, pending a breakdown by Chertoff of how he plans to spend the money. It is due in early December, after the midterm elections.

Asked whether Homeland Security would build 700 miles of fence, department spokesman Russ Knocke would not say. Instead, he noted that department leaders announced last month that they will spend $67 million to test a remote-sensing "virtual fence" concept on a 28-mile, high-traffic stretch of border south of Tucson over eight months, and then adjust their plans.

"We plan to build a little and test a little. ... Stay tuned," Knocke said. "We're optimistic that Congress is going to provide the department with flexibility."

The split between GOP leaders hungry for a sound-bite-friendly accomplishment targeting immigration and others who support a more comprehensive approach also means that the fence bill will be watered down when lawmakers return for a lame-duck session in November, according to congressional aides and lobbyists.

The office of Sen. Kay Bailey Hutchison, R-Texas, Thursday released a letter from House Speaker Dennis Hastert, R-Ill., and Senate Majority Leader Bill Frist, R-Tenn., promising to ensure that Chertoff has discretion over whether to build a fence or choose other options. Homeland Security officials must also consult with U.S., state and local representatives on where structures are placed.

The letter was inserted in the Congressional Record on Friday night because Congress ran out of time to reach a final deal, aides said.

"State and local officials in California, Arizona, New Mexico and Texas should not be excluded from decisions about how to best protect our borders with their varying topography, population and geography," Hutchison said in a statement added to the record.

Congress also hedged on when a fence would be completed. The law mandating it said Homeland Security officials should gain "operational control" of the border in 18 months. But the law funding it envisions five years. Chertoff has set a goal of two to three years, but only after completion of an immigration overhaul.

Staff writer Peter Baker contributed to this report