Here's why the GOP can't stop Trump

The mainstream Republican dream of stopping Donald Trump once the race is narrowed down to him and just one other candidate has a serious problem: For too many of the voters, Trump is their second choice.

A Quinnipiac University poll released Wednesday showed that in the national GOP horse race Trump led 43 percent, then Ted Cruz at 29 percent, and John Kasich with 16 percent. Thus, in order for Cruz to catch up with Trump, he would need virtually all of Kasich's supporters to flow to him.

But when respondents were asked for their second choices, Kasich's supporters do appear to flow more to Cruz — but not by nearly enough. In a recalculated match, going one-on-one with Cruz, Trump gets 46 percent support to Cruz's 37 percent. (This suggests that out of Kasich's 16 percent, 8 points went to Cruz, and 3 points went to Trump — but again, Cruz needs practically all of them.)

At this juncture, at least, Kasich's supporters don't make the difference between Trump and Cruz — because Cruz isn't close enough to Trump for their numbers to matter.

Likewise, Cruz's supporters were asked for their second choice — and in their case, Trump was a genuine winner. In the recalculated match, Trump reached an outright majority with 56 percent support, against 25 percent for Kasich. (This suggests that from Cruz's 29 percent, 13 points went to Trump and 9 points to Kasich.)