When President Donald Trump declared a national emergency on Friday morning, he justified the move with multiple lies. He downplayed the idea that most drugs enter through ports of entry (they do). He dismissed the notion that illegal border crossings are at historic lows (they are). He suggested a wall would keep out asylum-seeking migrants (most of whom are turning themselves in at the border).


At one point during Trump’s press conference, he was directly quizzed on where he gets the numbers that form the basis for his claims. He replied: “I get my numbers from a lot of sources, like Homeland Security primarily.”


As luck would have it, the person who is in charge of “Homeland Security” is coming to Capitol Hill very soon. Secretary Kirstjen Nielsen is scheduled to testify before the House Homeland Security Committee on March 6th, at a hearing entitled, “The Way Forward on Border Security.”


This gives Democrats a major opening to press DHS secretary Nielsen on many of the claims that Trump has been making about the border. And this has new relevance, now that those claims are the basis for Trump’s declaration of a national emergency, which he’s using to appropriate the power to build large portions of his wall without Congressional authorization.


If handled properly, questioning from Democratic members of the House Homeland Security Committee will put Nielsen in an exquisitely difficult position. That’s because Nielsen will have to choose between falsifying widely available facts, on the one hand, and undercutting Trump’s case for a national emergency, on the other.


“Members will be able to present Secretary Nielsen with clear facts and see whether she - under oath - will help to undermine his case for circumventing Congress, or if she will herself lie to cover up his lies,” Tom Jawetz, an immigration analyst at the Center for American Progress, told me.


Nielsen will have a very tough time propping up Trump’s lies, if she decides that’s what she wants to do. Take, for instance, Trump’s claims about drugs and the border. As Philip Bump points out, many members of Trump’s own administration — including some at the Department of Homeland Security — have themselves confirmed that most drugs cross through official ports of entry.


Is Nielsen really going to contradict that? If so, Democrats can simply quote those very same officials telling the truth, making Nielsen’s efforts to prop up Trump’s falsehoods look even more buffoonishly absurd.


Democrats say that Trump’s justifications for his national emergency will be one focus at the upcoming hearing. “When the Secretary comes before the Committee next month, she will need to defend and explain the Administration’s border security policies - as well as the President’s emergency declaration,” Adam Comis, a spokesman for Rep. Bennie Thompson, D-Miss., the chair of the committee, told me.


Surely the hearing will be focused on many other matters, such as Trump’s family separations, migrant child deaths at the border, detention policies, and a lot more. But if Democrats can carve out a bit of time to focus on Trump’s emergency justifications, that could prove important.


The text of Trump’s national emergency declaration itself cites “criminals, gang members, and illicit narcotics” to make the case that the border situation “threatens core national security interests and constitutes a national emergency.” In trying to support the idea that illegal crossings have gotten worse, it cites asylum-seeking families.


Democrats can hopefully get Nielsen to admit to multiple truths undermining Trump’s case, including the facts that most drugs cross through ports of entry; that research undermines Trump’s frequent claim that undocumented immigrants bring disproportionate crime; that illegal crossings are at historic lows; and that those asylum-seeking families are (again) mostly turning themselves in.


If Democrats can draw blood at the hearing, it could matter. For instance, if House Democrats move to pass a resolution terminating Trump’s national emergency, that will set in motion a process that will force Senate Republicans to vote on it in the near future. Depending on timing, if the Nielsen hearing does blow up some of Trump’s lies in a very public fashion, that could conceivably put a bit more pressure on Senate Republicans.


On top of that, with lawsuits getting teed up against Trump’s national emergency, Jawetz points out that revealing Nielsen moments could “strengthen the case for the onslaught of litigation ahead.”


To be clear, it’s hard to say how much legal significance it will have if those justifications are effectively debunked at the hearing. As I noted earlier, it’s possible that it simply may not have legal relevance if Trump’s national emergency is basically premised on fiction. At the same time, if it is decisively unmasked as such, it just might end up mattering.


Beyond all this, if Democrats can use this hearing and other future oversight hearings to debunk Trump’s border falsehoods, it could further undermine Trump’s public and political case for his national emergency.


Trump obviously believes that declaring a national emergency makes him look like he’s taking decisive action on behalf of his wall in the eyes of his base. But outside that bubble, this whole mess could very well get a lot worse for him.


Greg Sargent is a columnist with The Washington Post.