I, too, share this fear:
[O]ne thing that's striking about the current Democratic field is how few tough political races they've won. Bill Clinton won a number of competitive races in Arkansas before entering presidential politics. However, the top three contenders for the Democratic nomination have a combined total of two wins in competitive general elections above the state legislative level (Hillary's Senate victory in 2000 and Edwards's in 1998; Obama's 2004 Senate victory came against Alan Keyes) and only three wins total (add on Hillary's re-election in 2006). Only one of those -- Edwards's win in 1998 -- came in unfavorable territory.
Right, Edwards' 1998 race in North Carolina is the only time any of these three have shown an impressive ability to beat Republicans. Even here, by 2004 polls seem to have indicated that taking sufficiently liberal positions to make an Edwards presidential primary campaign viable had rendered him unpopular in North Carolina and since then he's only moved farther left. It's interesting to note that Obama actually has been in several tough races, at various levels, one of which he lost, but all his difficult campaigns have been against other Democrats.
UPDATE: I quoted text with a slight error, the corrected version: "This post previously said the top three Democratic presidential contenders had won three times above the state legislative level. In fact, they've won four times. The error has been corrected above."