John Cassidy considers the evidence that Trump’s support is both real and from all parts of the GOP:

At the national level, a recent Quinnipiac University survey of Republicans and Republican leaners produced similar findings. Trump was ahead among voters who described themselves as Tea Party members or extremely conservative, but also among those who described themselves as moderate or liberal.

There are a couple other important things to be learned from this survey. Cruz, Carson, and Rubio are virtually tied for second place, but Cruz and Rubio’s support is not as evenly spread out across different groups of Republicans. While “very conservative” respondents make up the bulk of Cruz’s support, Rubio receives most of his support from the “somewhat” conservatives. Both have slight leads in these sub-groups. Rubio does worse with both the “very conservative” and moderate Republicans, and Cruz likewise lags behind among “somewhat” conservatives and moderates. Trump’s support is more evenly spread out, and his lead among moderates is much larger (16 ahead of Rubio, 15 ahead of Carson) than Cruz and Rubio’s leads with the “very” and “somewhat” conservatives over Trump (4 and 1 respectively). If one were to judge solely by these numbers, one would think that Trump was the usual relative moderate candidate who goes on to secure the nomination.

Just as Romney did in 2012, Trump is receiving decent support from the two blocs of conservatives while getting almost a third of the moderates and liberals, and in a nomination contest in which moderates make up roughly a third of the electorate (and in some states more than that) this gives Trump a significant advantage over the rest of the field. None of the other candidates is doing much to peel moderate Republicans away from Trump, and few of them are in a good position to try. Likewise, “blue-state” Republicans make up a huge part of the primary electorate, and they include many of the less religious and more moderate Republicans that have been gravitating to Trump this year. While it seems bizarre that so many moderates would rally behind Trump, it wouldn’t be so strange for a “blue-state” candidate with the edge among moderate voters to come out ahead in the nomination contest.