If these people had taken a forceful position at the start, Trump would never have gotten this far. He’s not a Republican - he’s a big government crony capitalist, and on top of that, he’s nuts. He shares none of the party’s objectives or ideals, either in the social or economic area, and they should have challenged him at the very beginning. The fact that Sanders voters are the group most likely to cross over to vote for Trump if he’s the candidate should tell you all you need to know.
But the GOP has let its brand be weakened through so many compromises and so much timidity that the party leaders really didn’t even have the guts to oppose the nut-job, media driven campaign of somebody entirely opposed to what are in theory the official GOP positions.
To which official GOP positions is Trump "entirely opposed."