My opinion at this moment in time is that Trump cannot be defeated as the Republican Party's nominee even at the convention. If the convention is brokered denying him the nomination after he has gained so much support I close my eyes and shudder as the power in the Republican Party, weak and ineffective, folds like a paper tiger fearful of the violence that is behind Trump.
The rise (and hopefully ultimate fall) of Trump is reminiscent to me of the rise to power of the Nazi state. One by one responsible power brokers in Germany at that fateful time caved and through the Enabling Act in response to the Reichstag fire (lit surreptitiously by Nazis themselves but blamed on Communists and, of course, Jews,) gave the elected Chancellor Hitler all the power that he needed to assume totalitarian control. The Nazi rallies at Nuremberg foretold the portents of doom.
Trump's rallies show us in living color the violent nature of their supporters' essence and the encouraging remarks by Trump himself who easily admits if he is challenged "rioting will break out" show his tolerance of it.
It is up to ALL of us to stand with those in the Republican Party who have the power to challenge the delegate count which ultimately selects the nominee. In this case, unknowing by most, the Party has enormous power to choose the nominee. Whether they have the will use it is surely up for conjecture. I personally doubt that they will.