I see two distinct groups/camps making noise about stopping Trump, other than the other candidates for obvious reasons.

1. Those would really want to vote for a true, principled conservative who holds a majority of the same ethics, morals and values as they themselves hold. They want a President to be someone they trust and someone who has honor, integrity and virtue. They are tired of voting for the GOP party and the poor choices of candidates, so they will vote their conscience. They want to stop Trump because he doesn't appear to share the same core beliefs and is only pretending to be a deep conservative.

2. The locked in, ingrained, establishment Republicans who live to play politics and retain power in DC, even when they lose elections. Their goal is to wield power, even if the minority party, and win re-election as they talk tough on camera but vote soft. The establishment wants a candidate they can control and handle as necessary. They dislike the tea party, grassroots conservatism that has been brewing because it calls for things to radically change in DC in how things are done. Trump is a lone wolf and unpredictable and uncontrollable as a political power and how he will ultimately stand on the issues is a question... so he is a major threat to their scheming system in multiple ways.

(This same group wants to stop Cruz too, but they actually know what he will do and what he wants to do and what he stands for--and that is precisely what they do not want to ever see happen because it means a reform of the party and the gov't system which equates to a loss of their power and influence which makes them vulnerable in elections). So they are eagerly hoping for a brokered convention where they can run a moderate RINO that has mass appeal to defeat Hillary if possible...and if they lose, they will get to publicly blast the Dems and be tough occasionally while working with them in private for 4 years because they are scared of any negative media.

I'll add a 3rd group:

3. The Democrats. They want the power regardless of who runs. Trump is a handful with lots of virtues that are dislikable, so they see his lead and ae making a little noise now. But if Trump is the nominee, they will eventually bring the barrage of claims when it is time. To be expected.