WASHINGTON » Carefully crafted legislation that would end the government’s bulk collection of Americans’ phone records is under fire after the White House requested last-minute changes that critics say would water down its protections.

A year after Americans learned from National Security Agency contractor Edward Snowden that the NSA was secretly collecting vast amounts of telephone and email data, the House is preparing to vote this week on legislation intended to curtail domestic spying.

Although the bill is likely to pass Thursday, the changes hammered out in secretive negotiations over the last few days between the Obama administration and leaders on Capitol Hill have led some privacy groups and civil libertarians to withdraw their support. They warn that the revisions, including changes to what sort of government data searches would be permitted, could provide loopholes that would allow massive data collection to continue.

"I think it’s ironic that a bill that was intended to increase transparency was secretly changed," said Rep. Zoe Lofgren, D-Calif., a member of the House Judiciary Committee, which crafted the original legislation. "And it was altered in worrisome ways." She said she was unsure how she would vote.

So far, the changes appear modest enough to avoid tanking the bipartisan support needed for passage. But meddling with the accord poses inherent risks in a divided Congress where lawmakers have grown increasingly wary of intelligence operations. An unusual political alliance of liberal Democrats and small-government conservatives has thwarted earlier efforts to expand spy agencies’ reach into Americans’ private lives.

Many of those lawmakers remained undecided Wednesday, suggesting the final vote could be closer than the White House would like.

The White House insisted Wednesday that the changes were intended to meet the shared goal of the president and Congress to clip the vast collection of bulk "metadata," while ensuring against new directives that would impede routine investigations or efforts to combat terrorism.

Administration officials argued in the closed discussions, often held in the third-floor Capitol suite of House Majority Leader Eric Cantor, R-Va., that the bill’s language originally approved by the judiciary and intelligence committees was drafted too narrowly and could limit non-bulk data collection operations.

"There was no effort to soften the ban on bulk collection," said National Security Council spokeswoman Caitlin Hayden. "Our engagement was to ensure that the language of the USA Freedom Act would not have any unintended consequences for routine individual investigations."

Under the proposed legislation, the Justice Department and intelligence agencies would no longer be allowed to collect from telephone companies vast amounts of so-called metadata, including the times and lengths of calls but not the contents of conversations. Instead they would need to narrow searches by making "specific selection" requests based on certain criteria.

The bill would also require the government to seek a Foreign Intelligence Surveillance Act court order for any such requests — with an exception for emergencies that would allow data collection up to seven days before approval must be sought.

But at the request of the White House, the definition of "specific selection" was changed. Initially, a search would have been required to uniquely describe a "person, entity or account."

The White House complained that the definition was too narrow and would impede even routine investigations. The new language more broadly defines "specific selection" as a "discrete term, such as a term specifically identifying a person, entity, account, address or device."

Mark Jaycox, a legislative analyst at the Electronic Frontier Foundation, said the addition of the words "such as" make the new language "too expansive" and would allow "for much broader orders than privacy advocates are envisioning."

The American Civil Liberties Union said the new bill "leaves much to be desired."

In the Democratic-controlled Senate, which is considering its own version of the bill, similar bipartisan objections have been raised.

The top Republican on the Senate Intelligence Committee, Sen. Saxby Chambliss of Georgia, said he was concerned about the new version, and Sen. Patrick J. Leahy, D-Vt., chairman of the Senate Judiciary Committee, said he would seek to restore certain provisions.

Those familiar with the negotiations said the lawmakers won key concessions from the administration in return for the changes — including the appointment of advocates who can review FISA court decisions.

The government would also be required to "promptly" destroy any material collected that was deemed irrelevant to an investigation rather than be allowed to retain it indefinitely.

The bill also contains liability protections for companies that furnish information to the government, and says those firms can be compensated for their services.