Even before a new survey from Monmouth University was released on Monday, Ben Carson's slide in Iowa was apparent. The Real Clear Politics polling average in the state showed his spike and tumble in clear terms.

The new Monmouth poll, though, shows just how much he's slipped. In the overall horse race, Carson lost more than half of his support, dropping to fourth place behind Ted Cruz, Donald Trump and Marco Rubio. Cruz, who has been surging in the state, gained 15 points since Monmouth's late October poll. Monmouth changed its poll sample between the two polls, so the graph at right below shows the change in the same group between October and the new survey. Remember this; we'll come back to it.

Cruz now leads among the conservative groups that once backed Carson and leads among men. Only among women does Cruz not lead; there, Rubio has an advantage.

How much has Cruz benefited from Carson's slump? Here's how the vote among evangelicals changed since October. Cruz picked up nearly all of what Carson lost, with Rubio getting the rest. Trump didn't move.

This is not a good poll for Trump, for a few reasons. The first is that point above: He doesn't seem to have picked up much of Carson's support. But, second, there's the new sample Monmouth is using. Thirty percent of the people surveyed this time are folks who say they will caucus with the Republicans in February, while the rest have actually voted in Republican primaries. Those 30 percent are presumably less likely to actually go out and participate in the caucus -- and they're much more likely to back Trump.

Among regular Republican primary voters, though, Trump is in fourth place.

As The Post's Monkey Cage noted earlier Monday, this is part of Trump's strategy: Turn out less-frequent voters in order to give a big push to victory. That's possible, given Trump's celebrity and unorthodox campaign to date. That strategy is not a new one, though, and often is to electoral politics what land invasions of Russia have been to geopolitics -- a plan that leads to humiliating defeat.

If the election were tomorrow, though, the dynamics of this race would be dramatically different than what they are now because there would have been lots of ads and field work identifying voters and so on. Anyway, Cruz is in good shape as it stands.

Correction: This post originally described 70 percent of the pool of respondents as having participated in a caucus before, as opposed to a primary.