After some blow out victories in recent caucuses, the Sanders campaign seems to be reinvigorated. Polls had been showing Sanders running about even with Clinton. More recently, it looks like he may be running 5 points ahead.
Sanders continues to make life difficult for Clinton, even though it looks very unlikely that he could ever pass Clinton in the delegate count. Although Sanders may win Wisconsin, Clinton maintains solid leads in some of the following large State primaries, such as New York, Pennsylvania, and California. Sanders needs to average over 67% in all the remaining contests in order to pass Clinton, which seems highly unlikely. Still, if he can continue to stretch his lead in Wisconsin, it may help his momentum in some of the remaining States.
I don't think Sanders ever seriously thought he could be President. Perhaps he is emboldened by the level of success he has reached. But his primary goal of forcing Clinton to address important issues related to wealth disparity has already been made. Sanders, a man who for years thought the Democratic Party was too conservative to join, really carries too much baggage to become President. Although head to head polls show him doing well against potential Republican nominees, that is before the Republican attack machine has even started in on him. His support for socialist causes, his desire to raise taxes significantly on large portions of the population, and his past backing of Communist governments would all fall upon him with full force in a general election. He knows that as well as anyone.
Sanders will continue to keep Clinton on guard, but despite any additional successes, Clinton will be the nominee and remains the stronger choice for a victory in November.