Barack Obama's march towards the Democratic presidential nomination has picked up support from four more superdelegates.

The news pushes him ever closer to victory over Hillary Clinton - even as their primary marathon staggers on.

She added two superdelegates herself - party grandees - in what has become the last big contest as their race winds to a close.

The superdelegates are elected officials and party insiders who can vote as they choose at the party's nominating convention in August.

There are just 217 delegates to be chosen in the final six primaries, and neither candidate can win enough of them to claim final victory.

Meanwhile, 265 additional delegates have yet to be claimed, and their support will be the deciding factor. Though Mr Obama added to his lead in Tuesday's primaries, most uncommitted superdelegates want to remain on the sidelines.