Despite a boost from Barack Obama — but not the absent Hillary Clinton — Democratic senators today failed to stop the Bush administration from winning legal immunity for telecom companies that helped the government eavesdrop on Americans.

Obama voted with 30 fellow Democrats to allow the telecom companies to face lawsuits, which civil liberties groups consider a crucial chance to unearth information on the administration's programme of wiretapping without a court warrant.

But the immunity survived, with 18 Democrats crossing over to support George Bush.

Clinton missed the vote on a day of intense last minute campaigning as Virginia, Maryland, and the capital all held presidential primaries. Whether her absence will spark criticism from liberal Democrats, who have made the fight against so-called "telecom immunity" a high priority, remains to be seen.

The next challenge for Democrats is reconciling the Senate's wiretap bill, slated for final approval today, with a version backed by the House of Representatives that contains no telecom immunity and stronger protections for US citizens who could be caught in the surveillance net.

"Holding all the Democrats together on this, we've learned a long time ago, is not something that's doable," the party's Senate leader, Harry Reid, told reporters today.

The current authorisation for administration wiretapping without warrants expires on Friday, and Democrats are pursuing a temporary extension to allow more time for negotiations.

But George Bush, appearing confident that Republicans have the upper hand in the debate, has hinted he would veto such a move.

Opponents of telecom immunity already are prodding Democratic leaders to adopt the House's approach, effectively daring Bush to kill the wiretap bill that his own administration requested.

"The Senate has buckled," Democratic senator Russ Feingold said. "We are left with a very dangerous piece of legislation."

Kevin Bankston, a senior attorney at the non-profit Electronic Frontier Foundation (EFF), appealed directly to House Democratic leader Nancy Pelosi.

"It's time for Speaker Pelosi to draw a line the sand and make clear to the president that this House of Representatives is never going to pass any bill that includes immunity for lawbreaking telecoms," Bankston said in a statement.

The EFF is representing a group of Americans who are suing AT&T in a California federal court, alleging mass interception of their private communications without a warrant. Granting immunity to AT&T would almost certainly halt that lawsuit.

Other attempts to strengthen the wiretap bill's protections for US citizens also fell short today, but the telecom immunity vote was the most closely watched.

Obama made the most of his time on Capitol Hill, chatting up undecided senators whose votes at this summer's Democratic convention could help him defeat Clinton.

"There's no replacement for either of our presidential aspirants to be on the floor, talking to members," Democratic senator Dick Durbin, a close Obama confidante, said. "Senator Obama had the chance to talk to quite a few."

But many of Obama's colleagues had their own political futures to worry about.

Eight of the Democrats who voted to shield the telecom companies hail from Republican-leaning states, including several facing strong re-election challenges next year. Republicans are using the possibility of a future terrorist strike on the US as a cudgel in the wiretapping debate, accusing Democrats of wanting to give terrorists the same legal rights as American citizens.

In fact, Democratic senator Ron Wyden added extra protections for Americans to the wiretap bill, requiring a court warrant for surveillance of US citizens while they are overseas.

"The reason that the immunity issue is such a flashpoint is that the administration said for years and years, 'The [wiretapping] programme is legal'," Wyden said. "Then, after lawsuits were filed, they come back and say, 'Even though we told you the programme was legal, we need immunity'. It's very hard to square those two arguments."

Democrats are conscious that a White House-backed wiretap bill they passed in August alienated liberal voters and caused consternation within their ranks. They appear more prepared this month to counter what they characterise as deliberate misrepresentations by Bush and Republicans.

"This is fear-mongering, it is wrong, and it has obscured what is really going on here," Feingold said yesterday.

John McCain, the Republicans' likely presidential nominee, also cast votes today in the Senate, siding with his party to preserve telecom immunity and pass Bush's preferred bill.