"Trump has roughly about 40% nationwide of those republican votes at the moment(according to his own polls), but if the Republican party says: This guy is not our official presidental candidate because of reason XY, AND Trump says: I'll be a non party presidential candidate then, aren't the Republicans in trouble, because they will lose a huge amount of votes?"
Independents can make a difference to the party they draw votes from, but rarely win - they simply don't get seen as credible when it comes to the polling booth.
Trump may be an outlier in that regard?