WASHINGTON - As a sweeping immigration overhaul bill moves forward in the Senate, Republican efforts to ratchet up its border security measures threaten to upend a bipartisan compromise reached over four months of negotiations.

"If, in fact, the American people can't trust that the border is controlled, you're never going to be able to pass this bill," Sen. Tom Coburn of Oklahoma, the top Republican on the Senate Committee on Homeland Security and Governmental Affairs, told four officials from the Department of Homeland Security during a hearing Tuesday. "So you're going to have to help us figure out how to do it."

Sen. Marco Rubio of Florida, a Republican author of the bill whose support is considered essential to win over conservatives, said in a statement Tuesday that, for the bill to be passed, "it will have to be improved to bolster border security and enforcement even further."

Several committees are scrutinizing the legislation this week. The Senate Judiciary Committee will begin considering changes on Thursday. On Tuesday night, committee members submitted hundreds of amendments.

Under the bill, most of the 11 million people in the country illegally would be able to become citizens after 13 years, provided the Border Patrol can stop 90 percent of those trying to enter the United States without proper documentation in the most high-traffic areas of the border with Mexico.

The bill calls for the department to hire 3,500 more Customs agents and employ more border fencing, cameras, drones and radar systems to detect illegal crossings.

Senators from both parties challenged homeland security officials on Tuesday to explain how they would spend more than $4.5 billion in the bill for improving border security.

"You're going to get this money; what are you going to do with it?" Sen. Thomas Carper (D., Del.), chairman of the Senate Homeland Security Committee, asked the border security officials.

Border Patrol Chief Michael J. Fisher said that he would like to improve the agency's ability to move agents between areas of the border and its use of the intelligence collected by cameras, radar, and other surveillance technologies. Getting to 90 percent effectiveness rate in the high-trafficked areas is possible, he said. Right now the Border Patrol has between 78 percent and 85 percent effectiveness in those areas.