President Bush today signed into law a bill that authorizes construction of 700 miles of new fencing along the U.S.-Mexican border, a measure he hailed as "an important step in our nation's efforts to secure our border and reform our immigration system."

Before signing the Secure Fence Act of 2006 in the Roosevelt Room of the White House, Bush said the new law would help modernize the 2,000-mile border. But he said more must be done to achieve immigration reform, and he repeated his call for a program that would allow U.S. employers to bring in guest workers and would offer legal status to many of the estimated 12 million illegal immigrants nationwide.

Congress approved the Secure Fence Act last month as majority Republicans sought to show voters they were tough on illegal immigration ahead of midterm elections. The legislation did not include Bush's guest worker and legalization proposals, which were blocked by key House Republicans who insisted on first tightening control of the nation's southern border.

There was no immediate explanation of why the president waited nearly four weeks after passage to sign the fence bill. Some Democratic critics charged that the White House wanted to hold the signing ceremony closer to the Nov. 7 elections to give Republican candidates a boost.

A leading House Democrat, Rep. Silvestre Reyes of Texas, who served with the Border Patrol for more than 26 years before running for Congress, charged today that the bill "represents the worst in election year politics." Reyes, a member of the House armed services and intelligence committees, called it "an empty gesture for the sole purpose of sending a false message about the security of our nation."

While the new law authorizes 700 miles of double-layered fencing, among other measures, it does not include funding for the project. A separate $34.8 billion homeland security spending bill signed earlier this month includes $1.2 billion -- described as a "down payment" -- for various installations including the fence. The fence alone, stretching across parts of Texas, New Mexico, Arizona and California, is estimated to cost at least $6 billion to build.

Shortly after passing the Secure Fence Act, lawmakers practically ensured that the fence would never be built as advertised when they passed the spending bill, Washington Post staff writer Spencer S. Hsu reported.

That bill gave the Bush administration leeway to distribute the money to a combination of projects including roads, technology and "tactical infrastructure" to support the Homeland Security Department's preferred option of a "virtual fence." Thus, the $1.2 billion would pay for an unspecified blend of fencing, vehicle barriers, lighting and technology such as ground-based radar, cameras and sensors.

In addition, Republican congressional leaders pledged in a late-night session shortly before recessing 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."

Nor is it clear when any fencing would be built. The Secure Fence Act requires Homeland Security to 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 an overhaul of immigration law.

In his statement before signing the bill, Bush said it "will help protect the American people."

Illegal immigration has risen because "unfortunately, the United States has not been in complete control of its borders for decades," Bush said. "We have a responsibility to address these challenges. We have a responsibility to enforce our laws. We have a responsibility to secure our borders."

He touted a doubling of funding for border security under his administration, from $4.6 billion in 2001 to $10.4 billion this year, and a strengthening of the Border Patrol from about 9,000 agents to more than 12,000.

With the Secure Fence Act, "we're modernizing the southern border of the United States so we can assure the American people we're doing our job of securing the border," Bush said.

The act "is part of our efforts to reform our immigration system," but "we have more to do," he said. "Meaningful immigration reform means that we must enforce our immigration laws in the United States." Most businesses want to obey the law against hiring illegal immigrants, he said, "but they cannot verify the legal status of their employees because of widespread document fraud. So we're creating a better system for verifying documents and work eligibility and in the meantime holding people to account for breaking the law."

He renewed his call for a "temporary worker plan" to provide employers with foreigners willing "to do jobs Americans are not doing."

Bush added, "We must face the reality that millions of illegal immigrants are already here. They should not be given an automatic path to citizenship. That is amnesty. I oppose amnesty. There is a rational middle ground between granting an automatic path to citizenship for every illegal immigrant and a program of mass deportation, and I look forward to working with Congress to find that middle ground."

Some congressional Republicans have opposed Bush's proposal on grounds that it amounts to a de facto amnesty, even if Bush does not call it that. Amnesties in the past, such as a 1986 immigration law signed by President Reagan, did not give illegal immigrants an "automatic path to citizenship." Instead, they were required to go through a lengthy process that included determining their eligibility for the amnesty, applying for legal permanent resident status and waiting for five years after obtaining it before becoming eligible to apply for citizenship.

In a statement, Reyes, the House Democrat from Texas, said of the signing ceremony, "The administration and the Republican leadership are trying to provide a false sense of security with a press event touting a Band-Aid approach."

Instead of building up to 700 miles of new fencing, he said, "we need a comprehensive plan that addresses the three main priorities of the Border Patrol: manpower, technology and infrastructure."