With a comprehensive immigration bill in the dustbin, Senate Republicans sought Wednesday to win passage of its most popular piece, a $3 billion plan to improve security along the U.S.-Mexico border.

The budget-busting GOP measure would be added — over White House opposition — to a pending bill to fund the budget for the Homeland Security Department.

Republican sponsors such as Sen. Judd Gregg of New Hampshire said securing U.S. borders is as important a priority as fighting terrorism and the war in Iraq.

The move comes in the wake of the collapse in the Senate of President Bush's immigration plan, a compromise combining the popular border security initiative with a deeply controversial plan to legalize as many as 12 million unlawful immigrants.

The White House has already threatened a veto of the underlying homeland security bill for breaking Bush's budget and Gregg said the White House opposes the border security plan offered by Senate Republicans.

"The administration position ... is that they oppose it," Gregg said.

During the immigration debate last month, proponents of the broader approach such as Bush and Sen. Lindsey Graham of South Carolina argued that splitting up the compromise immigration package and passing border enforcement first would doom the broader effort.

But Graham said Wednesday that the public won't accept more controversial elements — such as a plan to give million of illegal immigrants a way to earn U.S. citizenship, dubbed "amnesty" by opponents — until the porous border with Mexico is made more secure.

"Border security is the gate that you must pass through to get to overall comprehensive reform," said Graham, who is up for re-election next year and facing political heat at home for backing Bush's unpopular immigration plan.

The emergency border security funding proposal is similar to one Republicans tacked onto an immigration measure to garner more GOP support for the bill, which died last month.

Democrats had supported that move — an infusion of $4.4 billion in mandatory funding — as a way of drawing broader backing for the compromise bill.

But it also includes several provisions that Democrats said went too far, such as allowing law enforcement officers to question people about their immigration status, cracking down harshly on people who overstay their visas, and imposing mandatory prison sentences on illegal border crossers.

For their part, Senate Democrats mulled their options on the GOP plan, which requires 60 votes to pass the 100-member Senate since it would be financed through additional debt. Democrats didn't immediately signal a willingness to kill the plan outright.

"On first glance, there's some stuff in this proposal we can support, but much of it also appears to be pretty objectionable," said Jim Manley, spokesman for Majority Leader Harry Reid, D-Nev.

One option under consideration is a Democratic countermeasure that would blend border security funding with bipartisan provisions to allow more foreign agricultural workers into the United States and to permit some immigrants who have grown up in the U.S. to become citizens if they go to college or serve in the military.

The GOP move is reminiscent of the successful plan last year to pass a bill to build vehicle barriers and 700 miles of fence on the southern border aimed at keeping out illegal immigrants from Mexico and other countries.

That effort was passed under GOP control of the House and Senate after the Senate passed a comprehensive immigration plan and House GOP leaders countered with the border fence initiative.