Donald Trump is not going to be the Republican presidential nominee. His unfavorable numbers among Republicans are too high for that, and Republican voters are too sensible. He is not going to be a huge threat as a third-party candidate in November 2016, since he will first have gone through the humiliation of losing the Republican primaries.
He is not going to pull the Republican nominee too far to the right. The other Republican candidates already know that most Republican voters want the federal government to stop illegal immigration, and that a large number of them want it intensely. The candidates would say most of the same things they are saying about that topic if Trump did not exist.
He is not going to render the Republican brand toxic. Voters will be able to distinguish between him and the eventual party nominee. Only left-wing partisans think he is a spokesman for the party.
Given all of this, it seems to me that the correct response by the other candidates, and by other anti-Trump Republicans, is not to denounce him ever more loudly or to try to change the debate rules to exclude him or to treat him with “kid gloves” (as a misleading headline in The Hill puts it). It’s to ignore him. Don’t get distracted from the fundamental tasks of a campaign, don’t give him the attention he wants, don’t get dragged down to his level. In a debate, just outclass him — which should be feasible.
Above all, trust Republican voters to see through him. They will.