From Jonathan Bernstein Bloomberg:
There are three ways that Donald Trump still could be denied the Republican nomination.
He could lose the delegate lead: Trump is ahead of Senator Ted Cruz by a little less than 300 delegates. That’s a large advantage, but there are still more than 800 bound delegates remaining to be selected, almost all of them in winner-take-all or winner-take-most states. There isn’t enough polling data to indicate what’s going to happen in most of these.
And that delegate lead is a little shakier than it might seem.