Barack Obama is scrambling to put his presidential bid back on track after denouncing his former pastor.

Obama's move comes as he and Hillary Clinton push onward in a nomination struggle that is dividing the Democratic Party.

With key primaries in Indiana and North Carolina looming next week, their fight has given Republican nominee-in-waiting John McCain weeks to unite his party and define his candidacy with few major challenges from the opposition.

At a news conference, Obama denounced Rev Jeremiah Wright whose comments and highly publicised appearances have threatened to sink his historic bid for the White House.

While he holds an apparently unassailable lead in elected delegates, the Wright controversy had created a heavy drag on Obama's momentum.

His refusal to sever ties to the theologian was seen as part of the reason Clinton turned in a nearly 10-point victory in Pennsylvania last week.

She has used her performance there to argue that the party's key superdelegates should back her as the most electable Democrat in the November general election.

There are about 800 Democratic superdelegates, officeholders and party officials who can vote for either candidate regardless of the results of state primary and caucus contests. Clinton has a narrow 21 superdelegate lead while Obama has outdistanced Clinton 1,729.5 to 1,595.5 overall.

With only nine state and territorial contests remaining, Clinton cannot achieve the 2,025 delegate count needed for the nomination without capturing most of the superdelegates who remain uncommitted.

That would put the party hierarchy at odds with Democratic voters and could further deepen the Democratic split.