Barack Obama headed into twin primaries in North Carolina and Indiana looking for a strong showing to help seal the Democratic presidential nomination.

Dual victories would all but knock Hillary Clinton out of the race. Polls, however, have found a small edge for her in Indiana. Mr Obama remains the favourite in North Carolina, although his lead has shrunk.

But there was little expectation that the primaries would settle the battle. Both Mrs Clinton and Mr Obama predicted they would still be campaigning in June.

Mrs Clinton needs to win at least Indiana to remain viable in the contest given Mr Obama's solid lead in delegates who will choose the nominee at the Democratic Party's convention in August in Denver.

Mr Obama meanwhile, needs a strong showing to erase doubts about his ability to win the November presidential election against Republican John McCain.

He has been on the defensive for weeks, losing the important Pennsylvania primary last month and struggling to distance himself from incendiary remarks by his former pastor.

Mrs Clinton has been chipping away at Mr Obama's advantage in North Carolina and the race in Indiana remains tight.

Surging fuel prices have played a big part in the campaigns, stunning Americans who are accustomed to the cheapest in the developed world. "I believe we should start standing up for the majority of Americans who are paying the outrageous gas prices," Mrs Clinton said. "I'm ready to take on the oil companies."

North Carolina and Indiana cannot mathematically settle the nomination. A candidate needs 2,025 delegates to win, and Mr Obama had 1,745.5 to Mrs Clinton's 1,608 heading into the latest contests.

The key to the nomination is held by superdelegates, party leaders and elected officials who are not bound by the outcome of state contests. About 220 superdelegates are still undecided.