Sen. Bernie Sanders, I-Vt. (AP Photo/Jacquelyn Martin)

Update: Right on cue, a Sanders spokeswoman on Tuesday morning informed reporters that they shouldn't even ask questions about the Islamic State. We are re-upping this post from Monday, as it is now even more apropos.

One day removed from the Paris terrorist attacks last month, the three Democratic candidates running for president gathered in Iowa for a debate. The first question was, predictably, about the attacks and what they meant for both the ongoing fight against the Islamic State, also known as ISIS, and the broader battle against terrorism.

Bernie Sanders got the first crack at it. Here's what he said:

Together, leading the world, this country will rid our planet of this barbarous organization called ISIS.

I'm running for president, because as I go around this nation, I talk to a lot of people. And what I hear is people's concern that the economy we have is a rigged economy. People are working longer hours for lower wages, and almost all of the new income and wealth goes to the top 1 percent.

He devoted a grand total of one sentence (!) to the attacks. He then launched into a 123-word riff on the necessity to address climate change, economic inequality and campaign finance reform.

That moment was a glimpse into the serious limitations that Sanders has as a candidate -- and why, as the race has turned to issues outside his wheelhouse, he has struggled. This from WaPo's John Wagner:

Terrorism and gun violence have dominated the headlines in the aftermath of the Paris terrorist attacks and mass shootings in Colorado Springs and San Bernardino. But the Vermont senator is sticking largely to a script that has nothing to do with either — emphasizing income and wealth inequality instead, the same issues that generated an unexpected groundswell of support for him over the summer.

Sanders’s near-silence on foreign policy and gun control were hard to miss at a time when the 2016 presidential race has come to be dominated by issues of national security and terrorism. They are not easy subjects for him, given a mixed voting record in Congress on gun restrictions and a noninterventionist foreign policy that he has chosen not to make a centerpiece of his campaign.

Sanders is -- sorry Sanders people! -- surprisingly one-dimensional as a candidate. When he is talking about the differences between the haves and the have-nots, about the need for more economic fairness, why we need to reform the campaign finance system or work to address global warming, he is terrific. When he is talking about anything else, he is, um, not.

Sanders built a movement in the early days of the race on the passion he exudes from every pore when talking about economic inequality. The contrast between Sanders's "people powered" campaign and the top-heavy, corporate feel of Hillary Clinton's effort was striking. And Sanders is, without question, closer to the true heart of the Democratic Party than Clinton on the vast majority of domestic issues.

The problem for Sanders is that external events changed the conversation in the race, and he has been unwilling or unable to change with it. Talking about economic inequality in the midst of a national debate about gun control and national security won't lose Sanders the ardent supporters he already has. But it will badly hamstring his ability to grow beyond the supporters he already has in what is, essentially, a one-on-one race with Clinton at the moment. (Sorry Martin O'Malley!)

For Democrats looking for a candidate who can stand up to the almost-inevitable Republican charge that the party of Obama is both unaware of the full scope of the fight against terror and unwilling to do what it takes to win that conflict, Sanders has done virtually nothing to convince people he can be that guy.

Being one-dimensional is okay -- and often actually a good thing -- as a senator. You need to find a pet issue of a pet palette of issues and then dig in deep on them. Almost every senator operates like this; they have a place (or a few places) where they are passionate and they seek to spend most of their time and energy there while sort of finessing the other stuff.

Being one-dimensional is very much not okay at the presidential level. The challenges the country faces -- both domestically and internationally -- are legion, and an ability to rapidly shift from one area of focus to another, no matter where your true passions lie, is essential.

Sanders has shown little ability or inclination to grow beyond his pet issue set. And, as a result, his campaign's challenge to Clinton has stalled.