Senators clashed Thursday over proposals to secure the U.S.-Mexico border contained in a comprehensive immigration reform bill, revealing divisions between Republicans and Democrats that could determine the fate of the most sweeping overhaul of the nation’s border laws in nearly three decades.

On the first day of debate over amendments, senators on the Judiciary Committee zeroed in on how well the 844-page legislation, developed by a bipartisan group of eight senators, is able to stem illegal immigration along the Southwest border.

As the debate began, several Republicans lobbied for a change to the bill, proposed by Sen. Charles Grassley (R-Iowa), that would require federal law enforcement authorities to establish operational control of the entire border for six months before allowing any of the country’s 11 million undocumented immigrants to earn legal status.

Democrats objected to the provisions, arguing that the bill already includes significant new border control measures and that the additional requirements would delay for too long the ability for illegal immigrants to earn green cards and citizenship.

“This is a confidence-building measure,” said Sen. John Cornyn (R-Tex.), who supported the amendment. “Border security, if it does not work as advertised, we would have failed in our responsibility to solve the problem.”

Democrats on the committee, along with two Republican allies who participated in developing the legislation, countered that the federal government has invested record amounts of money and manpower on border enforcement, helping to reduce the amount of people trying to cross illegally.

Sen. Richard Durbin (D-Ill.) noted the government spent $18 billion on border security in 2011.

“That’s more than we spend on all other federal law enforcement combined,” he said.

The committee rejected the Grassley amendment by a vote of 12-6, with all 10 Democrats and two Republicans rejecting it. But the debate foreshadowed potential problems for the comprehensive bill as it makes its way through the Senate and, potentially, the GOP-controlled House.

Many Republicans have said they will not support a path to citizenship for illegal immigrants unless they are convinced the border is fully secured.

The committee meeting Thursday marked the first of what is expected to be several days of hearings on amendments over the next three weeks. Senators have filed 300 potential changes to the bill.

Sen. Marco Rubio (R-Fla.), a member of the bipartisan group who is not on the Judiciary Committee, has said that the legislation will not pass the Senate or House without changes to the border control provisions.

“I want to know how the bill doesn’t repeat mistakes we’ve made in the past,” said Grassley, the ranking Republican on the committee who has been skeptical about parts of the legislation. “Since we only do comprehensive immigration reform about once every 25 years, or at least successfully so, we have to get it right . . . so that we don’t have to revisit this issue again.”

The amendments to the border provisions include requiring more reports by federal agencies, adding a second layer of fencing, requiring more border agents and giving Congress more authority over the federal implementation of border control plans.

Sen. Charles Schumer (D-N.Y.), a member of the bipartisan group, called the bill a fair one that represented compromises on all sides. As the committee began its meeting, Schumer said: “Don’t make an effort to kill a bill that is the best hope for immigration reform we’ve had in the country, and the best hope to break the partisan gridlock that has strangled the Senate, the Congress and the country.”

The Judiciary Committee will consider amendments to other portions of the bill at additional hearings scheduled for next week. The panel’s chairman, Patrick J. Leahy (D-Vt.), has said he intends to hold a committee vote by early next month in hopes of sending the bill to the full Senate in early summer.

The House is weighing whether to consider a comprehensive bill or break it into smaller pieces, a tactic opposed by Democrats and the Obama administration.

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