Wednesday night's Democratic debate on Univision crystallized something important: The discussion on immigration within the Democratic Party is further to the left than it has ever been.

But you wouldn't have picked that up simply from watching the messy, confusing Univision debate itself. And even if you did pick it up, it wouldn't tell you anything about whether Hillary Clinton or Bernie Sanders is the better candidate for Democratic voters who care about immigration.

After several debates in which immigration was mentioned only briefly or not at all, the Univision debate was a huge missed opportunity. It didn't do the thing that a primary debate is supposed to do: It didn't tell us anything about how a Bernie Sanders administration and a Hillary Clinton administration would handle immigration differently.

Here's what the debate showed — and what the real question for the candidates is now.

The Democratic Party has changed fast on immigration

Most of the immigration portion of the Univision debate — like the immigration portions of previous debates — was taken up by Sanders and Clinton attacking each other over things the other candidate had said or done in the past that seemed inconsistent with their current support of immigrants. For example:

Sanders voted against comprehensive immigration reform in 2007.

Clinton opposed driver's licenses for unauthorized immigrants in 2007.

Sanders implied that unauthorized immigrants drove down the wages of American workers.

Clinton said she'd send back children fleeing violence in Central America.

There's a lot here, and there's more where that came from. But honestly, the same would be true of most other Democratic politicians who've been active in national politics for the past 20 years.

Most center-left Democrats sounded like Hillary Clinton, attempting to balance support for comprehensive immigration reform with tough rhetoric on border security and immigration enforcement. Most progressive Democrats sounded like Bernie Sanders, attempting to balance support for the human rights of immigrants with deep skepticism about the effects of immigration on American labor.

The lesson you should take away from these records is less anything about Sanders or Clinton in particular than the fact that the Democratic Party mainstream has moved rapidly to the left on immigration over the course of the Obama administration.

The focus shifted from Congress to the White House

One crucial change is that Democrats have stopped seeing immigration as an issue with several equally important moving parts. Protecting unauthorized immigrants who have been in the US for years, especially those who have US citizen children, has become the paramount Democratic priority on immigration reform.

That, combined with congressional paralysis (and the fact that Republicans have run screaming from immigration reform as Democrats have embraced it), has shifted the center of gravity on policymaking to the executive branch — which is responsible for the day-to-day of immigration enforcement.

So while both Sanders and Clinton promise to pass "comprehensive immigration reform" through Congress (Clinton promises to introduce a bill during the first 100 days of her presidency), a legislative agenda isn't the centerpiece of either candidate's immigration platform. The focus is on what they would do, as executives, to protect unauthorized immigrants while waiting for some hypothetical day in which Congress will be open to comprehensive reform.

The 2016 consensus: Only deport immigrants with criminal records

Given that focus, one of the most important immigration moments of the campaign came Wednesday night when Jorge Ramos got both Sanders and Clinton to explicitly promise that they would not deport children, and then, even more broadly, that they would not deport immigrants who had not committed crimes.

That's a direction both candidates have been moving throughout the campaign. Sanders released a platform last fall that promised to protect up to 9 million unauthorized immigrants from deportation. Clinton hasn't released a detailed immigration platform, but has made public remarks promising to expand President Obama's executive actions to protect immigrants from deportation.

By extracting those promises, Ramos was doing the same thing he did in 2008, when he got then-candidate Barack Obama to promise that he'd introduce a comprehensive immigration reform bill in his first year in office. Ramos was creating a benchmark against which to set the actions of a Sanders or Clinton administration — something that he, other press outlets, and activists could use to condemn Sanders or Clinton if they fell short.

The fact of the matter is that it's extremely likely either Sanders or Clinton would fall short of that promise — because it's an extremely difficult promise to implement. And in the context of a primary campaign, where voters have a choice between two people making the same promise, the question is: How would they accomplish that promise? And which of them can be more fully trusted to be able to keep it?

Fulfilling this promise would be very, very hard

If you wanted to oversimplify a little (but only a little), you could sum up the Obama administration's record on immigration enforcement in a single sentence: It is really hard to control, perfectly, who gets deported and who doesn't.

The federal government's capacity to deport unauthorized immigrants on a regular basis is greater than it's ever been. So for the Obama administration, the question has been whom that capacity is used to deport. (Arguably, this is also the way Clinton's immigration policy would work; Sanders's immigration platform has promised to dismantle many of the policies that make widespread deportation possible to begin with.)

But most of the decisions about who gets deported are made on the ground, in Immigrations and Customs Enforcement or Border Patrol field offices. Many of the people running those field offices — and many, many of the agents staffing them, not to mention the unions representing those agents — don't agree that they should only be deporting certain kinds of unauthorized immigrants and leaving others alone.

Left to their own devices, they're going to deport some of the people the president would prefer not to deport. And the more the White House and political appointees give them top-down directives, the more they rebel.

President Obama spent several years butting his head against this wall. Ultimately, he and his administration decided to take things out of the hands of immigration law enforcement agents by using executive action to protect immigrants preemptively.

Right now, that's what both Sanders and Clinton are promising to do too. Right now, that promise works. But the Supreme Court is going to rule this spring on whether Obama violated the Constitution by granting such broad protections, rather than allowing immigration officials to make case-by-case decisions.

How easy it is for President Sanders or President Clinton to implement their immigration promises is going to depend hugely on what the Supreme Court rules this year. It's entirely plausible that they're either going to have to come up with alternative ways to grant protections from deportation or return to the Obama administration's past strategy of simply telling agents not to deport people.

Either of those is going to require a president who is both very progressive and willing to expend meaningful amounts of time and political capital on legal and bureaucratic fights. It's going to require a tremendous amount of commitment from the executive branch — more than either Sanders or Clinton has shown on the campaign trail so far.

Would either candidate really do this stuff?

The important question for voters who care about immigration to ask themselves, then, is whether either candidate would either fulfill these pledges as president.

We've seen over the decades that Hillary Clinton is a relatively cautious politician. On the campaign trail this go-round, she's said most of the things immigration activists have asked her to say. But it's hard to trust that fundamentally transactional promises — like the promise she made to Ramos during the Univision debate — are promises that will be kept if political circumstances change.

Immigration activists will probably have some influence in a Clinton White House. But that influence will likely be balanced against many other considerations of political feasibility and broad priority setting.

Already, the immigration fact sheet on her website relies on wiggle words like "everything possible under the law." Given the pending Supreme Court case, that's in part a measure of simple prudence. But it also means that when the rubber hits the road, her specific policies may end up conflicting with the very ambitious goals she outlines.

Sanders, meanwhile, has adopted an extremely progressive immigration platform but at times seems personally unfamiliar with its contents. Last night, both candidates fielded questions from a woman whose husband had been deported and who wanted candidates to promise to help reunite their family.

Sanders's platform (unlike Clinton's) calls for using a device called humanitarian parole to accomplish exactly what the woman wants — but when answering the question, he didn't mention that. I've seen Sanders talk about his platform with more familiarity, but not Wednesday night.

There are a few issues that are top of mind for Bernie Sanders — income inequality, higher education, campaign finance reform — and immigration is not one of them. The fact that it wasn't top of mind even during the Univision debate raises questions about when it would be top of mind in a Sanders presidency.

If fulfilling his pledges once in office were as simple as checking a box or flipping a switch, that might not matter. But in reality, Sanders would have to overpower a set of recalcitrant and outspoken public-employee unions who'd be supported by Congress and substantial segments of public opinion. Would Sanders really let a huge portion of his first year in office be consumed by this controversy, or would he set it aside in order to focus on his battle against plutocracy?

To a certain extent, these aren't questions that can be settled in a debate. But they represent the real distinction between the candidates on immigration right now. They're saying the same things. Which one is more likely to grasp the magnitude of what they're committing to?