In the 2004 Presidential election, many republicans decided they were fed up with George W. Bush and decided that they would support and vote for John Kerry, the democratic candidate. Some even decided to start a website that is still up, (but not active)
here.
So why is it such a stretch that Obama can also attract some republicans, especially considering that Obama is arguably more charismatic and a better orator than Kerry and that our situation has gotten worse?
Sure, there are some republicans who are voting in democratic primaries simply to vote against Hillary, or to select who they think will be the weakest candidate in the GE (some think that will be Obama, others think that will be Clinton). That shouldn't take away from the fact that there are republicans who genuinely are attracted by Obama's message and will vote for him in the general election. Besides, registered republicans have only been a small fraction of the vote in the democratic primaries.
Merely painting all republicans with a broad-brush misrepresents the situation at hnad and is disingenuous considering that many of those criticizing Republicans for Obama 2008 were touting Republicans for Kerry 2004.