Barack Obama's hopes of emerging as the first black Democratic presidential nominee could now depend on top party officials intervening to stop the fight with Hillary Clinton in the name of unity.

The focus of the Democratic race has shifted to Indiana and North Carolina where Mr Obama is favoured to win in primaries early next month. But victory in these states will still leave him short of winning the nomination and allow the damaging battle with Mrs Clinton to continue through the summer. Defeat could be fatal for his chances.

One idea being floated is for a so-called Gang of Four - the former presidential hopeful John Edwards; the former vice-president Al Gore; the Democratic Party leader, Howard Dean; and the House Speaker, Nancy Pelosi - to intervene after the primaries on 6 May. They would then call on the superdelegates to make a commitment within 10 days so that the party can have a nominee before the August convention in Denver.

At that stage, the Obama campaign hopes to have won 2,025 delegates, enough to secure the nomination. Mrs Clinton could still thwart him if she persuades enough superdelegates to back her on the grounds that she is a tougher, more effective candidate who would carry more states in the presidential race.

If Mr Obama cannot break through the submerged racial prejudice of older white voters, the argument of many Democrats goes, he cannot win the White House. Time magazine's commentator Joe Klein said poorly educated white voters had grave doubts "about a young, inexperienced African-American guy with an Islamic-sounding name and a highfalutin fluency with language".

Despite her solid victory in Pennsylvania on Tuesday, Mrs Clinton faces an uphill struggle in Indiana where Mr Obama has a slight edge in the polls. One advantage that Mr Obama has going into the race is that Indiana's population is concentrated in smaller cities such as Gary, where he has a high profile. He is also expected to take North Carolina, a southern state with a large black population.

However, Mrs Clinton is now waging a battle to persuade the remaining uncommitted superdelegates that his consistent failure to defeat her in big states such as Pennsylvania, New York and California makes him a weaker candidate for the presidential election in November.

Mrs Clinton raised enough doubts about Mr Obama in Pennsylvania to persuade white working-class voters to hand her a comfortable nine percentage points victory. A big part of Mr Obama's problem is race. The exit polls in Pennsylvania revealed that 20 per cent of voters think race is important and heavily backed Mrs Clinton.