A highly controversial wiretapping bill headed toward a conference showdown late Tuesday after the Senate passed a version giving legal immunity to phone companies that took part in President Bush’s terrorist surveillance program.

With a 68-29 vote that flatly contradicted the House on this key issue, the Senate passed White House-backed legislation updating the 1978 Foreign Intelligence Surveillance Act (FISA). 19 Democrats sided with their GOP colleagues, including Intelligence Committee Chairman Jay Rockefeller (D-W.Va.). The legislation would grant retroactive immunity to carriers that participated in the National Security Agency’s (NSA) surveillance program — which bypassed the Foreign Intelligence Surveillance Court — as long as they show they had authorization from the federal government.

But a difficult conference could stall final passage of the bill, as lawmakers continue to argue over whether to shield the carriers from the 40 or so lawsuits they face relating to the program.

Differences between the Senate version and the House-passed FISA bill, which does not include the immunity provision, must be resolved by Friday, when a six-month-old interim FISA law expires.

Under pressure from the White House, Congress passed legislation last August that granted broad powers to the government to conduct warrantless surveillance, including on Americans, as long as the communication concerned a party reasonably believed to be abroad.

In a sign of the bumpy road ahead, House Judiciary Committee Chairman John Conyers Jr. (D-Mich.) wrote the White House Tuesday to reject the administration’s claim that the immunity is justified by secret documents it has shared with him and other select members of Congress.

“There is no basis for the broad telecommunications company amnesty provisions advocated by the administration and contained in the [Senate FISA] bill,” Conyers wrote White House Counsel Fred Fielding. “These materials raise more questions than they answer.”

But senators rejected a series of six amendments earlier Tuesday that would have reined in or strengthened oversight of the NSA program. They included amendments that would strip or revise the immunity protections, as well as language that would bolster the role of the FISA court, which has traditionally overseen foreign-intelligence gathering on U.S. soil.

The sole Democratic victory, adopted by voice vote, was an amendment by Sen. Sheldon Whitehouse (D-R.I.) that would prevent the government from preserving any surveillance it accidentally collects on Americans in the course of targeting someone else.

Even if House and Senate negotiators can agree, final passage faces other obstacles. Sen. Chris Dodd (D-Conn.) has threatened a filibuster if the final bill provides blanket immunity for the phone companies, while President Bush has threatened to veto any bill that does not.

Senate Minority Leader Mitch McConnell (R-Ky.) urged a quick conference with the House and said he doubted a conference was even necessary.