Mr Obama on Friday picked up nine more superdelegates, including one who previously backed Mrs Clinton, in an apparent outpouring of new support as he seeks to become the first black US president.

Superdelegates - party insiders and elected officials who are not bound by state results - will likely cast the deciding votes at the party's August nominating convention because neither candidate will have enough delegates to clinch the nomination from state contests alone.

The two Democrats have been lobbying superdelegates to line up behind them in the final push for the nomination. While Mr Obama has presented himself, albeit subtly, as the inevitable candidate, Mrs Clinton's efforts have been to ward off further defections and convince the crucial voting block that her candidacy still retained signs of life.

Mrs Clinton also gained a superdelegate.

After a strong showing in two state primaries earlier this week, Mr Obama appeared to be more convincing in delivering his message. The new endorsements put him within just a half-point of catching Mrs Clinton in endorsements from superdelegates.

Mr Obama currently has 271 to Mrs Clinton's 271.5. Little more than four months ago, on the eve of the primary season, she held a lead of 169-63.

New Jersey Rep. Donald Payne, who announced his decision on Friday, is one of at least 10 superdelegates who have switched allegiances from Mrs Clinton to Mr Obama. None have publicly switched the other way.

Mr Obama also picked up the endorsement of the influential American Federation of Government Employees union on Friday.

In an interview with National Public Radio, former Democratic candidate John Edwards - who has declined to endorse either former rival - said Mrs Clinton has made a compelling case for her candidacy, but 'I think it's very hard for her now to make a compelling case for the math. I mean, I think that's the reality of what she's faced with. She knows that.'

Both candidates were campaigning on Friday in Oregon, which holds its primary on May 20. While polling in the Pacific Northwest state has been sparse, Mr Obama is believed to hold a significant advantage over his rival.

But that race - one of six remaining contests - is unlikely to decide a 16-month nomination battle that has both polarized and riveted Democratic voters. It is mathematically impossible for either candidate to clinch the nomination without the support of superdelegates.

As of late Thursday, Obama had 1,859.5 delegates to Clinton's 1,697. Obama is just 165.5 delegates short of the 2,025 delegates needed to win it.

The roughly 800 superdelegates are the ones likely to decide the outcome. Of that total, about 260 remain uncommitted. About a third of the undeclared superdelegates are members of Congress. - AP