Supporters of Democratic presidential candidate Sen. Bernie Sanders, I-Vt., attend a rally in Washington in June.

Now that Senator Bernie Sanders is effectively dropping out of the presidential race and endorsing Hillary Clinton, what's Donald Trump's best strategy for courting the former Sanders voters? I've already made the case that Trump can grab a very small but still significant number of Sanders' supporters simply because he is now the only "change" candidate still standing. And Trump is pulling no punches today, already expressing his "surprise" and accusing Sanders of selling out by joining the "rigged system."

That kind of sharp rhetoric makes sense because the one kind of Sanders voter Trump is most likely to have a shot at co-opting is the angry voter who believes both parties have sold out a large segment of the public for decades. These angrier voters will also tend to be a bit older than the average Sanders backer, who tended to be younger and more idealistic as opposed to being "angry."