Charleston, West Virginia: Hillary Clinton pinned hopes on a convincing win in the West Virginia primary to raise doubts about Barack Obama's electability, but her rival was already mapping out his strategy for the upcoming presidential campaign against Republican John McCain.

Clinton had every reason to expect a big victory over Obama after the vote yesterday in West Virginia but the chances of turning around her bid to become the first female US president seemed all but over. Polls show her leading by as much as 40 percentage points in a state that has large numbers of working-class whites - a group that usually backs the former first lady.

Obama may be only a few weeks from clinching the Democratic nomination, no matter what happens in West Virginia or in another Clinton stronghold, Kentucky, a week later.

In his tone, words and itinerary, Obama is focused on McCain almost to the exclusion of his fading Democratic rival. He planned to spend primary night in Missouri, a bellwether in the general election.

Superdelegate boost

Only 28 delegates are at stake in West Virginia. Obama has already picked up 26 superdelegates since last Tuesday, when he routed Clinton in North Carolina and narrowly lost to her in Indiana.

Obama, who is vying to become the first black president, is now mathematically within reach of clinching the Democratic nomination by the end of the primary season on June 3 - even if he loses half of the remaining five states and in Puerto Rico.

Neither Obama nor Clinton can reach the 2,025 delegates needed to secure the nomination just through the final contests. That means the superdelegates - the nearly 800 party leaders and elected officials who may vote for whichever candidate they choose regardless of the state primary or caucus results - are essential to capturing the nomination.

Clinton's best chance to slow Obama is to buy herself more time by moving back the finish line. She will get that chance on May 31 when the Democratic National Committee's rules panel considers proposals to seat delegates that had been stripped from Florida and Michigan after they violated party rules by holding their primaries too early.

If the delegates are reinstated that would increase the number of delegates needed to get the nomination to 2,209.

Clinton would also cut into Obama's delegate lead since she won the primaries in both states. But even under the best scenario for Clinton, Obama would still be left with a lead of about 100 delegates, with fewer than 250 superdelegates left to be claimed.

"We need to do well everywhere," said chief Clinton strategist Howard Wolfson. "Our hope is that superdelegates will look at the results in some of these states and recognise that Senator Clinton would be the best nominee against John McCain."

"This may be the most important vote you've ever cast," she told a crowd in Fairmont. "Let's have a huge vote in West Virginia."