No one was really asking for Bernie Sanders' opinion on the newest "birther" controversy, but he decided to offer it anyway. It turns out that, perhaps unsurprisingly, Bernie Sanders believes that birtherism of any sort is due to, you guessed it, racism:
"As I mentioned to you earlier, my father was born in Poland. Like Obama, I am the son of an immigrant father. Nobody has ever asked me for my birth certificate. Maybe it's because of the color of my skin being different than Obama's," Sanders said Monday to cheers from the students at Hoover High School in Des Moines, Iowa.
Now nobody, including Trump, is talking about the Obama birther issue anymore. However, he can't say that he thinks Donald Trump is a racist against Ted Cruz, because that would involve an implicit admission that Ted Cruz is not strictly white, which is something the Democrats have been studiously avoiding acknowledging for the better part of four years (at least). So instead Bernie Sanders has to resort to calling Trump a racist by making reference to an issue that has been dead for years.
Of course, as stupid as the entire Cruz birther issue is, there is at least a fundamental distinction between both Obama and Cruz's issues (which concerned the question of whether Cruz/Obama were born in the United States - and whether that matters) and his own, which is decided on the different (but equally solid) ground that he was born in the United States. Sure, there are people who argue that it should not be the law that so called "anchor babies" aren't automatically citizens, but no one is arguing that this isn't what the law currently says.
The real contrast here, of course, is the contrast between the way the Ted Cruz issue is treated versus the issue of how the exactly equivalent John McCain issue was treated - and boy, is John McCain white. If you wanted to accuse Donald Trump of racism based on his disparate treatment of birthers, that would be the comparison to make; however, that would also involve Bernie Sanders saying something nice about a Republican, so he can't allow it to stand.
A factual rejoinder to Bernie Sanders would also point out that Republicans never even considered Arnold Schwarzenegger for President, even at the height of his popularity, because of the undisputed fact that he was not a natural born citizen, even though he was white. The birthers of various stripes (and there are in fact also Rubio birthers and Trump birthers) are an interesting and perhaps excessively zealous lot, but as far as I can tell, they are not racist.
But facts have never gotten in the way of a liberal accusing someone of racism before.