The difficult road ahead for comprehensive immigration reform became more evident Thursday as Republican critics mounted a sustained assault on the legislation, demanding that it include considerably greater border security measures before legalizing any undocumented immigrants.

The contentious beginning of the debate in the Senate, where the bill’s prospects for approval are considerably better than in the House, was a clear signal of tough times as the legislation moves forward.

During a 71 / 2-hour hearing, the Judiciary Committee wrestled over 32 proposed changes focused on border security and control as the committee began a long and grueling amendment process that is expect to last weeks.

Proponents managed to resist the most significant changes as a majority of senators voted to reject proposals from GOP members that would require the government to build 700 miles of double-layered fencing and maintain complete operational control of the entire southern border before allowing illegal immigrants to gain citizenship.

As each proposal was defeated, frustration mounted and tempers flared among the most conservative Republicans.

See which amendments were adopted into the immigration bill

“The committee has voted down every serious border security amendment presented,” said Sen. Ted Cruz (R-Tex.), who lost his bid to scrap the legislation’s entire border-control section and replace it with his own. “This committee has consistently rejected any attempt to put real teeth in it, and if it does not have that, in my opinion, this bill will not pass.”

In all, the committee adopted 21 amendments, including eight offered by Republicans. Among them was a measure from Sen. Charles E. Grassley (R-Iowa), one of the fiercest critics of the bill, that expanded a requirement that the government apprehend 90 percent of people attempting to cross the border illegally from just high-risk sectors to the entire Southwest border.

Democrats, and two Republicans on the committee who helped negotiate the legislation, hailed the results as evidence that they were committed to a bipartisan process to improve the bill that represents the most far-reaching changes in the nation’s immigration system in three dec­ades.

They characterized the GOP border-security offensive as an effort to lard the bill with unattainable security measures and make it more difficult for the nation’s 11 million undocumented immigrants to gain legal status.

“Senator Cruz is opposed to a path to citizenship,” said Sen. Charles E. Schumer (D-N.Y.), one of the eight senators who negotiated the legislation. “No matter what we put in there in terms of border security, he cannot support any bill that has a path to citizenship. . . . Let’s not keep bringing up this false issue that we’re doing nothing on border security.”

Border control was the opening flash point in a debate over an 844-page bill that also contains new measures to increase visas for low- and high-skilled workers and eliminate some categories of family visas, which are likely to spark further clashes among Republicans and Democrats.

Judiciary Committee Chairman Patrick J. Leahy (D-Vt.) has said he hopes to finish the debate on up to 300 amendments by the end of the month and send the bill to the full Senate in early June. President Obama has thrown his support behind the legislation in hopes of avoiding the bitter partisanship that helped sink his efforts on gun control and deficit reduction.

Some Republicans insist that they have to be certain that border control is a priority in any legislation before allowing undocumented immigrants to earn legal status. That has opened a divide within the GOP as moderates and conservatives battle over what a secure border would look like.

The bill, developed by four Democrats and four Republicans, provides $7 billion in funding for additional fencing, aerial drones and border agents.

Many Republicans have argued that it is not enough, and Sen. Marco Rubio (R-Fla.), who helped negotiate the bill, has said he agrees that border provisions must be strengthened for the legislation to have a chance in the GOP-controlled House.

As the debate began Thursday, several Republicans lobbied for a change proposed by Grassley that would require federal law enforcement authorities to establish operational control of the entire border for six months before allowing undocumented immigrants to earn legal status.

“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 countered that the Obama administration has invested record amounts of money and manpower on border enforcement, helping to significantly reduce the number of people trying to cross illegally.

Sen. Richard J. 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 to 6, with all 10 Democrats and two Republicans rejecting it.

“I want to know how the bill doesn’t repeat mistakes we’ve made in the past,” said Grassley, who is the ranking Republican on the committee and 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 committee approved measures to require more reports from federal agencies charged with enforcing border laws, to eliminate fees on immigrants who cross the border legally at ports of entry, and to give the Department of Homeland Security more leeway in how to spend $1.5 billion on border control strategies.

Rubio, who is not on the committee, celebrated the adopted amendments in a series of press releases declaring that they will make the border provisions stronger.

But a proposal from Sen. Mike Lee (R-Utah) that would give Congress authority to approve or reject the homeland security agency’s plans on border control was rejected by proponents of the bill, who worried that such a measure would allow lawmakers to hold up the process indefinitely.

During a break in the proceedings, Schumer said he remained upbeat, but he acknowledged the skepticism of some Republicans.

“Obviously, some on the other side have less trust in the government to really seal the border,” he said. “Anyone who’s followed this issue knows this journey is always a perilous one.”

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