During the campaign, Donald Trump made what seemed like an outlandish promise at the time. “I will build a great, great wall on our Southern border and I’ll have Mexico pay for that wall," he said in his announcement speech.

Mexico already said it is not paying for the wall, and congressional Republicans haven’t been successful in passing funding either, but the Trump administration’s plans to build the Southern border wall are already well underway.

In late August, after a months-long proposal process, Trump’s administration announced four companies would begin building prototypes for the border wall this fall. Construction on eight prototypes on the Southern border has already begun — and is expected to be completed within 30 days.

The prototypes, which will be 30 feet tall and 30 feet long, will be used to identify which style of physical barrier is best fit across the various terrains on the southern border.

“The WALL, which is already under construction in the form of new renovation of old and existing fences and walls, will continue to be built,” Trump tweeted earlier in September, after reports of an agreement between the president and top Democratic leaders appeared to be yet another setback for funding the project.

Democratic minority leaders in Congress Nancy Pelosi and Chuck Schumer said Trump had agreed to support the Obama-era Deferred Action for Childhood Arrivals program in exchange for some border security — excluding funding the wall. But the prototypes have already been funded by Congress.

That doesn’t mean there won’t be many roadblocks along the way. Trump will still need Congress to give him a lot more money to get it done. How much more, we don’t know (although the prototypes should give a better idea). And it seems so far, if Democrats have anything to say about it, Trump’s promise of a “big beautiful wall” will continue to be met with a wall of resistance.

Trump’s administration is taking steps toward building the wall

In March, Customs and Border Patrol put out a call for proposals on the Southern border wall. By late August they awarded four contracts for companies to build border wall prototypes.

The awards went to Alabama company Caddell Construction Co (DE), LLC; Arizona company Arizona company Fisher Sand & Gravel Co., DBA Fisher Industries; Texas Sterling Construction Co., and Mississippi’s W. G. Yates & Sons Construction Company. Each company will have 30 days to complete their prototype, which will then be tested by border security experts for their durability, anti-climbing features and general aesthetics. The companies are expected to incorporate what would be the addition of increase security technology, like cameras, sensors, and lighting.

The idea is that these prototypes will “create a ‘design standard’ for operational walls," CBP has said. "The new designs would be added to our menu of existing designs, and allow us to tailor a specific wall design to the unique demands of individual areas of the border."

Each prototype is estimated to cost between $400,000 to $500,000, funded through the $20 million Congress authorized for the Department of Homeland Security earlier this year, and previously through the 2016 budget.

As Vox has explained, paying for a preliminary model of the wall would likely give more clarity to how much the actual wall would cost — as that remains a big unknown. Estimates so far have put the price tag anywhere from a low of $12 billion, which was floated by the Trump administration, to $15 billion estimated by House and Senate leadership to even $21.6 billion, the highest estimated cost in a proposal seen by the Department of Homeland Security, which would be about half DHS’s yearly budget. A report put together by Democrats on the Senate Committee on Homeland Security and Governmental Affairs estimated an even higher price tag for the wall and patrolling technology, at a whopping $70 billion — more than four times Trump’s initial figure.

Funding the wall in Congress is and will continue to be a challenge

Eventually Trump will need Congress to fund his border wall. And if wants that to happen, he’s going to have to be willing to shut down the government over it.

Republicans may have control of the House, Senate, and White House, but when it comes to funding the government, eight Democrats have to sign onto their spending bill to reach the 60-vote threshold in the Senate.

This has given Democrats an incredible amount of leverage in the congressional wall debate: Early on, Democrats identified the border wall as a tangible issue area to show resistance to the Trump agenda. If they, as a cohesive voting bloc, refuse to vote for any federal spending bill that funds border wall construction, the government shuts down.

So far congressional Republicans haven’t been willing to go so far — and Trump, at least for now, has acquiesced. That hasn’t stopped Republicans from proposing legislation that would fund border security — including physical barriers. Most recently Homeland Security Committee Chair Mike McCaul (R-TX) unveiled a $10 billion plan to fund Trump’s Southern border wall, a proposal that is unlikely to go very far in the Senate. Sen. John Cornyn (R-TX) also has a bill in the upper chamber that would allocate $15 billion toward border security, for physical barriers and security technology.

To date, Democrats have successfully postponed funding the border wall, first negotiating a 2017 government spending package that didn’t include wall funding, and most recently striking a deal with Trump himself to fund the government through December at current spending levels. In other words, there’s still no wall funding. After the Trump administration announced they would be sunsetting DACA, giving Congress six months to enshrine the policy into law, the White House again said wall funding could be negotiated separately — seemingly relinquishing yet another point of leverage against Democrats.

Republican leaders, who have already been reluctant to shut down the government over the wall, will likely be even more resistant to the idea closer to an election year in 2018. Not to mention that there aren’t that many Republicans in Congress that want to prioritize funding the wall in the first place (a recent USA Today survey found only 69 of the 292 congressional Republicans gave a clear endorsement of funding the border wall. Three Republicans opposed it, and the rest either “evaded a direct answer” or refused to respond all together.)

There’s a lot more consensus over funding “border security” or a “virtual wall” — which the White House has acknowledged is a more tangible goal.

"I've always thought the wall was a metaphor for securing the border," Sen. Ron Johnson (R-WI) said. "We have an administration that's committed themselves to securing the border, whatever shape and form that takes."

But that doesn’t change the fact that the Trump administration has been moving forward with construction for a physical barrier.

That work has already begun.

The wall faces more challenges than just funding

Say Trump gets the money, and he somehow figures out a way to build a wall through rivers, canyons, and jungles. He still will have a big legal problem on his hands, from environmentalists to American landowners to sovereign territory.

To build the wall, federal officials will have to take a sizable chunk of private land on the Southern border away from American citizens, many of whom have owned rights to the land for centuries. Make no mistake, this process would be incredibly time-consuming and expensive for the federal government, and a point of contention with American voters.

“This is a battle,” Rep. Henry Cuellar (D-TX), whose district covers 280 miles of the border, told the Los Angeles Times. “In Texas, we have a long tradition of private property rights. Any time big government starts using eminent domain and taking land — especially the valuable part, access to water — then it becomes a battle cry. Lawsuits will definitely be coming in.”

The federal government has the legal power to take private land through eminent domain, if it compensates the landowners for the property. But landowners also have the right to due process, which in the past has tied up the government in courts for years.

This has happened before. After the 2006 Secure Fence Act’s passage, officials began the long process of buying off people’s lands, and hundreds of landowners fought back. There are still more than 90 cases tied up in court a decade later, according to Terence Garrett, a border security expert with the University of Texas Rio Grande. So far, the government has spent nearly $80 million to compensate landowners to build the existing fencing. The Chicago Tribune reported that Customs and Border Protection “estimates that it will spend an additional $21 million in real estate expenses associated with the remaining condemnation cases, not including about $4 million in Justice Department litigation costs.”

In California and Arizona, the government already has the rights to most of the border land. But Arizona is also home to the sovereign territory of the Tohono O’odham Nation reservation, which has already come out against building the wall. If the government attempts to build the wall through the 75 miles of sovereign territory in Arizona, it won’t be anything short of a fight.

Then there are the environmental concerns. As Vox’s Eliza Barclay and Sarah Frostenson reported, “the 654 miles of walls and fences already on the US-Mexico border have made a mess out of the environment there.” This already reared its head over the wall prototypes after former DHS Secretary John Kelly allowed companies to waive the environmental review to expedite the process.

So far, it seems like the impact on endangered species is not of concern.