From the moment he made it his central campaign proposal to build a wall on the US-Mexico border (and make Mexico pay for it), Donald Trump signed up his hypothetical administration to fight for its construction. “Build the wall!” became a chant everywhere from campaign rallies to high school basketball games.

The wall arguably handed Trump his biggest win — the presidency itself. It could turn out to be one of his biggest failures.

The fundamental problem is that very few people in the Republican Party affirmatively want a literal, physical concrete wall across the US/Mexico border. And in fact, there are constituencies both in the party (like Texas border landowners and the lawmakers who represent them) and within the Trump administration’s orbit (like Border Patrol agents) who have good reasons to fight hard against such a thing.

But anything short of a literal, physical concrete wall threatens to disappoint two constituencies that also carry serious weight with the president: the Trumpist base, for whom the wall has become a symbol of making America great again, and cable news pundits, who’ll find it all too easy to needle Trump over backing down on his central campaign promise.

The administration pushed hard on including money for the wall in the funding bill to keep the government open, and even feinted toward allowing the government to shut down if funding wasn’t included — signaling Trump still intends to keep his promise. But the speed with which the president folded on the shutdown threat is a hint that he might have realized the sell will be a tough one, even with Republican majorities in both chambers.

But what we saw this week was only the faintest echo of the difficulties the administration might have once it actually tries to make the wall real.

Some uphill battles are worth picking in politics, either because they're important policies or because they galvanize one's base and split one's opposition. This is the sort of thing that threatens to do the opposite. If you thought the fight over health care showed that President Trump doesn't have a governing coalition, you haven't seen anything yet.

The closer the wall comes to reality, the more intra-GOP fights will metastasize over it

Let’s be honest: Many of the people who voted for Donald Trump — maybe even most of them — don’t actually care if a wall is built on the US-Mexico border or not.

Many of them never expected it to happen: A May 2016 Fox News poll found that a third of Trump supporters didn’t believe he’d actually build a wall. (Voters who were undecided at the time were even more skeptical; while some of those people probably voted for Trump in the end, it seems unlikely they’d be more convinced he’d keep his promises than his longtime supporters.)

Nor will they be all that torn up if he fails to go through with it. A late-April poll of four swing states Trump won — Florida, Wisconsin, Ohio, and Pennsylvania — found that 40 percent of them would be “disappointed” if the wall didn’t get built by 2018, while the other 60 percent would not.

Furthermore, many of the “disappointed” voters wouldn’t be that disappointed: Three-quarters of them said they’d still vote Republican even if Trump broke his promise on the wall. They were much less sanguine about other Trump campaign promises, like passing an infrastructure bill. (A University of Virginia poll of Trump voters found something similar: Three-quarters said building the wall was at least “somewhat” important, but only one-third said it was more important to spend money on the wall than on health care.)

Of course, 10 percent is not an insubstantial slice of the electorate. But nor is it tantamount to “the Republican base,” or the constituents of Republican legislators. For the voters Republicans care about, as a whole, the wall isn’t a big deal.

And then there are the constituents for whom the wall is a big deal — because they really, really don’t want it to happen.

Republicans who live within 350 miles of the border are much less supportive of the wall than Republicans who live further away. And those who live on the border, in many cases, are the least supportive of all — because they’re the ones whose land would have to be seized, via eminent domain, so that the wall could be built on it.

“In Texas, we have a long tradition of private property rights,” Rep. Henry Cuellar told Vox’s Tara Golshan. Cuellar is a conservative Democrat who’s supportive of more border security but wary of the wall. “Anytime big government starts using eminent domain and taking land — especially the valuable part, access to water — then it becomes a battle cry.”

This is not hypothetical. The government is still fighting at least 100 lawsuits from property owners over the last time it used eminent domain to seize border land, to build a fence under the Secure Fence Act. That litigation, in many cases, has been pending for over a decade.

So far, the lawsuits Trump has fought on immigration have been good fights — losing in the courts is bad for policy, but losing at the hands of the city of San Francisco and the “very, very liberal Ninth Circuit” is good for keeping the base angry.

The eminent domain lawsuits over the wall would be bad fights. The plaintiffs would be landowners, many of them Republicans — and, in theory, the very people Trump is trying to protect from lawlessness and criminality by building the wall to begin with.

And instead of dealing with the easily caricatured Ninth Circuit, he’d be dealing with some of the most conservative judges in America. Many of the still-pending Secure Fence Act cases are in the courtroom of Judge Andrew Hanen — who’s such a hawk on immigration that when Republican states were looking for their best chance to block the Obama administration’s Deferred Action for Parents of Americans program in 2014, his is the courtroom they picked.

Texas Republicans, especially in the House, are cognizant of this — they’re a lot more bearish on the wall than the Trump administration. And they’d likely be joined by libertarian Republicans skeptical of the government’s legal authority to seize private land — and deficit hawks skeptical of the government’s track record on border security, which is littered with abandoned or ineffective projects to the tune of billions of dollars.

Of course, there are certainly Republicans and conservatives who believe that no price is too high to pay for law and order. And Trump would likely have important validators when making the case for more border security: Border Patrol agents themselves. The border agents’ union has become increasingly vocal on politics and policy issues over the past few years, and it’s closely aligned with Trump.

But the border agents have their own opinions about what the wall should and shouldn’t be. The Trump administration, however, didn’t appear to ask the agents for their input when it put out its first notice that it would be soliciting contracts for a wall prototype: It specified that the prototypes would have to be for a physical structure made of concrete. The administration ultimately sent out two requests for proposals, one for a concrete wall and one for a wall of “some other material” — but it didn’t require that the other material be something border patrol agents could see through.

Many border agents agree that more physical barriers are needed at the border (though they acknowledge that the fencing that already exists in many urban areas along the border has been effective). But they see the wall as a complement to their own work, not a replacement. And they’re adamant that they be included in the planning process: “Get the input of the local Border Patrol agents that work in those areas,” Shawn Moran of the National Border Patrol Council told Vox, “not someone in DC who might just be making a judgment call for the entire border.”

If the Trump administration’s haste — or its desire to build the most imposing wall possible — leads to “someone in DC” settling on a wall plan that border patrol agents object to, Trump could lose a key political ally. It’s hard to imagine the administration succeeding in getting Congress to fund a wall that Border Patrol agents don’t want built.

The wall’s importance is symbolic. That’s what makes it so hard to compromise on.

There are plenty of ways for the Trump administration to ramp up border infrastructure without building a “big, beautiful,” opaque, imposing wall.

It could reinforce existing fencing. It could build new physical barriers along some segments of the border, to funnel border crossers into areas where it’s easier for agents to find and catch them. It could increase surveillance technology and call it a “virtual” or invisible wall — an option that some Republicans, like Sen. Lindsey Graham (R-SC), have been not-so-subtly pushing for some time.

But Trump didn’t promise any of these. He promised a physical wall. He promised a big, beautiful wall. And that’s what distinguished him from the rest of the Republican field, and won him a reputation among a certain segment of the Republican base as the man willing to do what it took to stand up to unauthorized immigration.

Those hardcore Trump supporters agreed with the candidate when he said that under President Obama, “we don’t have a border” — either ignorant of or simply ignoring the hundreds of miles of fencing and thousands of agents already stationed there, or the long-term trend of declining apprehensions. (Apprehensions have plummeted further under Trump — which might mean some of his policies are working to deter people from crossing the US/Mexico border, but further undermines the case that building a wall is necessary.)

And while some of those Trump loyalists might be willing to go along with whatever Trump does and call it a “wall,” some of the conservative leaders who helped fuel his rise might not. He’s already being accused of capitulating on the wall, simply by not agreeing to shut down the government over it. If he makes incremental improvements to the existing border infrastructure — the stuff he said was tantamount to having no border at all — they may very well call his bluff.

There is no policy case for a concrete wall across the entire 1,900 miles of the US-Mexico border. But there is a powerful symbolic case for it: It would send a message (not least to Mexico) that the US was reasserting its dominance and withdrawing from a globalized (or globalist) world into an America-first one.

That is why Trump’s promise to build a wall was so memorable. It’s why it was a good fight for him to pick on the campaign trail — it motivated his base and distinguished him from his opponents.

But it’s a fight that he now can’t avoid with a compromise. He’s gunning right for it. If Trump caves on the wall, Laura Ingraham might turn on him, while cable news pundits debate endlessly just how bad it is for Trump that he’s broken his core campaign promise.

Trump’s problem, though, is that his need to keep his own promises isn’t exactly a rallying point for Republicans in Congress or Republican voters. It’s not enough to get them to support a proposal that most of them don’t care about and some very much object to. It’s not enough, in other words, to hold the party together and avoid a very, very bruising political fight.