why trump is going to be a problem for the GOP even if he loses

I  don't think the GOP will nominate Trump. But in a sense, that doesn't matter. Trump on the sidelines is not going to be like other GOP losers, who gracefully make way for the winner and fall in line. Trump represents ideas - genuinely idiotic ideas. And whoever wins will either have to gingerly embrace them while denying them or simply deny them. In that case, Trump will be drumming for his ideas right there on the sidelines. So this man is a genuine problem for the GOP whether he wins or doesn't.