Barack Obama has dramatically narrowed Hillary Clinton's lead in the superdelegate race, after winning a slew of endorsements in the last week.

The Illinois senator picked up four in the past seven days, pulling to within 30 of Hillary Clinton's lead, according to an Associated Press tally.

As the Democratic primary season nears its end game, the competition for the superdelegates, who cast votes at the August nominating convention, becomes even more critical to the candidates' chances of victory.

Obama has steadily been eating into her lead. In December, according to an Associated Press tally, Clinton led Obama by 106 superdelegates. In February, her lead had been cut to 87. Now she leads by only 30.

Obama leads in the overall delegate count, with 1,634 to Clinton's 1,500. A total of 2,024 needed to win. But neither he nor Clinton, a New York senator, can clinch the nomination solely on the votes of the pledged delegates awarded in proportion to the results of the state contests.

About 255 of the 794 superdelegates have yet to publicly commit to a candidate. Those include party leaders like Nancy Pelosi, the speaker of the House, and former vice president and Nobel Prize winner Al Gore. Former president Jimmy Carter hasn't stated a definite preference, but signalled recently he would back Obama.

Since last Friday, Pennsylvania senator Bob Casey Jr, Minnesota senator Amy Klobuchar, Wyoming governor Dave Freudenthal and former US senator John Melcher, a Democratic national committeeman from Montana, joined Obama's corner.

Clinton's last major superdelegate announcement came on March 18, when powerful Pennsylvania congressman John Murtha gave her his nod.

Beginning in February, Obama racked up a string of victories in state contests, and with that momentum attracted more superdelegate endorsements than Clinton.

Although Clinton is ahead in opinion polls in Pennsylvania, where Democrats head to the polls on April 22, she must now deal with the possibility that superdelegates who have committed to her will defect to the Illinois senator, particularly if he puts up a good fight in Pennsylvania.

"It's a slow drip, drip, drip -- but it's dripping the wrong way," Joe Trippi, an advisor to former Democratic candidate John Edwards, told the Los Angeles Times. "Psychologically, they're playing defence with superdelegates, not offence."

Georgia congressman John Lewis, an early Clinton backer, switched to Obama at the end of February.

Over the weekend, Clinton supporter Emanuel Cleaver, a Missouri congressman, told Candian radio that he would be "stunned if [Obama] is not the next president of the United States.'' He insisted, however, that he wouldn't switch his allegiance.