Why do the Democrats persist in nominating candidates too liberal to be elected to the White House?

As we seem to march to nominate Obama, are we forgetting what happened with John Kerry, Dukakus, Mondale and McGovern? We generally succeed with moderates - Clinton & Carter (and arguably Al Gore) and fail spectacularly with liberal nominees. Personally I want to see the Party win, and am very tired of hearing Nancy Pelosi whine that Dems in Congress cannot be effective without a Dem in the White House - and we need progress.