The US president needs his own party's votes and two independents - but former Democratic senator Joe Lieberman has gone rogue

President Barack Obama has invited all Senate Democrats to the White House today as a new obstacle was thrown in front of hopes of seeing his health reform bill pass the Senate before Christmas.

The main source of a potential delay was Joe Lieberman, the former Democratic senator who is now an independent and who has threatened to join the Republicans opposing the bill as it stands.

Lieberman's move infuriated Democrats who believed last week that he had signed up for a compromise bill.

The White House spokesman, Robert Gibbs, said yesterday that Obama remained hopeful of the health bill being passed before Christmas. "He hasn't changed his view. And, look, we've heard causes and concerns for months. The president still believes the Senate is going to act," Gibbs said. Failure at the White House summit would almost certainly rule out passage before Christmas.

Although Lieberman is part of the 60-strong Democratic caucus in the Senate, he has become a hate figure for the left, in part because of his high-profile backing for Obama's presidential rival John McCain last year, but mainly because of the way he has dealt with health reform.

Democratic senators last week raised hopes that the bill would be passed after agreeing a compromise that dropped a government-run scheme to compete with private insurance companies. Lieberman had opposed the scheme.

To soften the blow to liberals, the Democrats suggested extending Medicare, a government-run scheme for the over-65s, to anyone 55 or over. But Lieberman now wants that removed too.

Tom Harkin, the Democratic senator from Iowa, reflected Democratic frustration yesterday when he said he had thought Lieberman had been fine with the Medicare proposal. Lieberman has in the past championed such a plan.

Lieberman's vote counts because the Democrats need 60 of the Senate's 100 votes to override blocking tactics, such as the filibuster. They have the votes on paper: 58 Democrats plus two independents. But the reality is that one of the independents, though part of the Democratic caucus, is Lieberman.

The Washington-based Politico website reported that the White House put pressure on the Senate Democratic leader, Harry Reid, yesterday to drop the Medicare plan to get Lieberman on side and the bill passed.

But a White House spokesman, Dan Pfeiffer, denied it. "The White House is not pushing Reid in any direction. We are working hand-in-hand with the Senate leadership to work through the various issues and pass health reform as soon as possible," said Pfeiffer.

Even if the Senate was to pass the bill before Christmas, there would still be a potentially lengthy process to follow, with the house and the Senate needing to negotiate to reconcile rival health bills before a final vote on common legislation, and that is almost certain to stretch into next month.