Think about it this way: It now looks very likely that Cruz will beat Trump in Iowa, at which point Carson’s campaign will be pretty much finished, and Mike Huckabee and Rick Santorum will disappear — and with Cruz suddenly ascendant, he’s likely to pick up their supporters, pushing him up to Trump-like levels in the national polls.

Once that happens, would you really bet on Trump to beat Cruz in many of the evangelical-heavy Southern states that vote in late February or early March? In Cruz’s own state of Texas? That seems unlikely.

And Trump has more support in the South than he does in the high plains or Mountain West. So to survive a Cruz ascendancy among evangelicals, he would need to dominate in states like New York, Pennsylvania, Michigan and Florida.

But if we treat New Hampshire as a plausible template for outcomes in those states, then Trump needs Rubio and Chris Christie and Jeb Bush to all stay in the race and for all of them to keep winning exactly 15 percent of the vote; then and only then would his 30 percent be sufficient to prevail.

That seems less than unlikely, indeed nearly impossible.