In late October, Quinnipiac University released a poll that showed an earthquake rumbling across Iowa: For the first time, Ben Carson was leading in the state. By Nov. 1, Carson was up seven points on Donald Trump in the Real Clear Politics polling average in the state.

And then the attacks in Paris happened, and Carson and Ted Cruz switched places.

That's a pretty remarkable graph, really — including that uptick at the end for Marco Rubio. The consistent story, as it is in New Hampshire, is the strength of Donald Trump.

In Quinnipiac's new Iowa poll, releasedMonday, Trump has a lead for the first time in a while. He's been top tier for months, but in Iowa — a state that has rewarded conservatives Mike Huckabee and Rick Santorum in the past two cycles — he has drifted in and out of the lead. As a testament to his durability, 61 percent of his supporters said they wouldn't change their minds before Feb. 1, compared to 55 percent of Cruz backers and 52 percent of voters overall.

Tracking the top four candidates in Quinnipiac's poll since October, you can see how things have changed. Carson sank out of sight. Cruz picked up most of his support from evangelicals and a huge amount of his support from conservatives.

So good news for Trump, right? Yes — but there are some warning flags.

One of the things that becomes important as the election approaches is the confidence voters have that their chosen candidate can defeat the opposing party in the general election. Most signs point to that opponent being Hillary Clinton, and Iowa voters are less convinced than they were last month that Trump is the right guy to pit against her. (Which is probably true.)

In fact, Iowans think Cruz and Rubio would both be about as successful against Clinton as Trump — suggesting that Cruz or Rubio might gain more support as voters make up their minds.

Trump's support is also higher among voters that have never participated in a caucus before. He sees this as a good thing — that he's energizing people to come to the polls. Maybe he is. But people who've never voted before have a 0 percent track record of getting to the polls, making relying on them to turn out awfully risky. (Notice below that Rubio gets a lot more support from would-be first-timers, too.)

Third, more than a quarter of Iowans say they'd never back Trump. That's the same as wouldn't back Jeb Bush, for what it's worth. But it's far higher than any of the other leading candidates.

The iceberg that sinks the mighty S.S. Trumptanic looks something like this. Over the course of the next three weeks, people get nervous about his ability to win a general, and he sheds support. His core supporters don't end up voting after all. If others falter, their support goes to someone besides Trump.

At the moment, Iowa looks like a toss-up. But things change.