Barack Obama illustrated this when he lurched upon Hillary's incomprehensible stance on a plan to give driver's licenses to illegal immigrantsin New York:
Barack Obama added: “I was confused [by] Sen. Clinton's answer. I can't tell whether she was for it or against it. One of the things that we have to do in this country is to be honest about the challenges that we face.”
As a former First Lady, Clinton should have known that what a state like New York does in regards to illegal immigration is irrelevant to the job of the Presidency, which is a federal position. If she had pointed this out while also saying that she was confident New York Governor Eliot Spitzer would make the right decision, she would have been able to close the book on the issue and avoid the trouncing that she received at the hands of her opponents.
Democratic party supporters and left-leaning people in general across the country were pretty depressed after the 2004 election, unable to understand why on earth an "idiot" like George W. Bush was able to trounce John Kerry, who, if you weren't aware, fought in Vietnam. With poll numbers that would normally spell defeat for any sitting president, it was obvious s that the cause of Bush's defeat was Kerry's all-over-the-place stance on the top issue of the election, Iraq.
If the Democrats pick another candidate who is unable to articulate a coherent message to voters, they will see the same results of narrowly avoiding victory that their party has repeated in presidential campaigns.