But when the target is Obama himself? Well, the same reasons people voted for him in the first place, despite knowing his complete failure to have any qualifications and policy positions at odds with most of America, kick in. He's "historic" and is a way for the public at large to pay penance for whatever racial sins they or their ancestors may (or may not) have committed. To admit that Obama is a complete and utter failure and disaster ruins that image, and so Obama the individual has to be defended even if the Obama administration requires critique.
Now, say what you like about the intelligence of the average Democrat politician (and I have), there's one thing they're good at: manipulating public perception. They know the above is true about Obama, so when their backs are against the wall, when not even hue and cry of a fictitious "war on women" is sticking, their final trump card is to claim that Obama is under attack from the GOP. Even if they have to make up the attack themselves.