In the wake of the firing of Homeland Security Secretary Kirstjen Nielsen on Sunday, NBC News reported her relationship with President Trump became tense in part because of one specific issue: Trump’s support for reinstating a policy of separating children from their families when arriving at the U.S. border with Mexico.

“A senior administration official said it seems Trump is convinced that family separation has been the most effective policy at deterring large numbers of asylum seekers,” NBC’s Julia Ainsley and Geoff Bennett report. It is in keeping with other reporting, including from The Washington Post, suggesting Trump wanted to take a harder line on the increase in migrants arriving at the border in recent months. After Nielsen was rumored to be a target of Trump’s ire last year, he reportedly took a more positive view of her leadership after Border Patrol used tear gas to repel a crowd, including families, trying to enter through a gap in a border fence.

There’s an obvious recent spike in the number of “family units” arriving at the border — groups that include members of an immediate family. During the period that the family separation policy was explicitly in effect last year — from April to June, shaded on the chart below — the number of family units apprehended at the border remained fairly constant. (There was also an uptick in unaccompanied minors in May, which the separated children effectively became when removed from their parents.)

After July, the number rose quickly.

It is not clear the extent to which the separation policy actually contributed to deterring families from arriving at the border, though, according to multiple administration officials, that was explicitly one of the points of the program. While the number of family units apprehended at the border held steady from March to July, that period includes a month after the policy was ended. What’s more, there were far fewer family units apprehended in those same months of 2017 when there was no explicit policy of separation.

What is clear, though, is the policy of pulling apart families at the border is one of the least popular Trump has proposed during his presidency. In June of last year there were several polls demonstrating how unpopular the policy was — even, relatively speaking, among Republicans.

In polls from both Quinnipiac University and CNN and its polling partner SSRS, about two-thirds of Americans opposed the administration’s policy. That included, in each poll, a third of Republicans — an unusually high level of dissent from the president’s own party.

Even among those who approved of Trump’s job performance overall in that CNN poll, nearly 3 in 10 opposed the family separation policy.

While Trump argues action is needed immediately to stem the influx of families at the border and, particularly, the number who then seek asylum in the United States., Americans haven’t been entirely convinced of that urgency. A Post-ABC poll released in January found only a quarter of respondents reported believing situation with illegal immigration at the border was a crisis, with about half calling it a serious problem. A January Quinnipiac poll found that 54 percent of the country said there was a security crisis at the border, a result driven by broad support of the idea with Republicans.

In the 2018 election, which Trump sought to make a referendum on immigration, only a quarter of voters identified it as their most urgent issue in casting a ballot.

Again, even setting aside the questions of morality and legality (which reportedly is what cause Nielsen to hesitate), there’s no proof that the separation policy is the only or best way to curtail families seeking entry into the United States. It appeals to Trump clearly in part because he sees it as demonstrating toughness, as using tear gas on migrants reportedly also did. It also probably appeals to him because he seemed to withdraw from the policy only reluctantly in 2018.

But for a president seeking reelection next year, it is another inexplicable decision, after his embrace of throwing out Obamacare last week. The policy is probably popular mostly with those who would vote for him anyway while being viewed as anathema by many others. (Two-thirds of independents disapproved of it in CNN’s poll, for example.)

It again brings to mind a quote from a Trump supporter in Florida after the government shutdown amplified damage done by hurricanes in the state last year.

“I voted for him, and he’s the one who’s doing this,” a government employee said to the New York Times about the president’s embrace of a shutdown. “I thought he was going to do good things. He’s not hurting the people he needs to be hurting.”

It seems that, in Trump’s eyes, Nielsen wasn’t either.