George Bush is poised for a major victory this week as Congress nears final approval of a plan to provide legal immunity to private companies that aided government wiretapping as well as expand those spying powers.

Debate on the wiretapping bill is slated to begin in the Senate today, with a vote expected by week's end. Although civil liberties groups and liberal activists have pressed Democrats to oppose the proposal, its approval is considered a near-certainty.

The bill's most controversial provision gives legal immunity to telecommunications companies that helped the Bush administration monitor phone calls and emails without a court warrant in the aftermath of the 9/11 attacks.

The immunity debate has created particular headaches for Barack Obama, who last fall joined a group of liberal senators in blocking a separate wiretapping bill that contained a liability shield for telecoms.

But after securing the Democratic presidential nomination, Obama veered to the centre and indicated he would support the wiretapping plan even if the final version cancelled lawsuits against the companies. His staunchest supporters on the left protested the sudden shift, even forming a network on Obama's website to castigate him.

Obama attempted to smooth over the rift in a statement posted to that online network yesterday.

"Given the choice between voting for an improved yet imperfect bill, and losing important surveillance tools, I've chosen to support the current compromise," Obama wrote to the backers disenchanted with his move.

"Democracy cannot exist without strong differences. And going forward, some of you may decide that my [wiretapping] position is a deal breaker. That's okay. But I think it is worth pointing out that our agreement on the vast majority of issues that matter outweighs the differences we may have."

The wiretapping plan passed the House of Representatives on June 20 by a 293-129 vote, with several Democrats from conservative states joining Republicans to make up the margin of victory.

The bill seemed to face an uphill battle as recently as February, when Democrats in Congress allowed the previous, Republican-written statute on government wiretaps to expire rather than give in to White House demands on immunity.

But the tide turned in favour of compromise this summer, with existing wiretaps set for de-authorisation in August and Democrats fearing election-year attacks on national security issues.

A deal was eventually reached between House Democratic majority leader Steny Hoyer and Republican leaders. Democrats won some concessions from the White House: the new plan prohibits warrant-free spying on Americans travelling overseas and requires a court to evaluate the basis for wiretaps before they go into effect – not after.

Still, the American civil liberties union (ACLU) and other leading civil-rights groups have condemned the bill, calling it a capitulation to Bush and an unconstitutional violation of privacy rights.

"Not only will the [wiretapping bill] allow for the wholesale violation of Americans' fourth amendment rights, it will shut the door on investigations into the administration's warrant-less wiretapping program by closing active court cases," Colleen Connell, executive director of the Illinois ACLU, said in a statement.

"The Senate is the last opportunity for any real improvements to be made to this legislation - senators should fix the bill or vote it down."