Donald Trump’s big, beautiful wall on the U.S.-Mexico border is at risk of crumbling before it’s even built.

For over three years, Trump has promised to construct a massive wall spanning thousands of miles of the southern border, and he’s insisted that the government of Mexico would pay for it. That promise was at the heart of his victorious 2016 campaign, but in the past two years, the Trump administration has failed to secure funding from Mexico — and also Congress — to pay for the border wall.

Now, Democrats are weeks away from taking control of the U.S. House of Representatives, and the new majority will almost certainly not approve any wall funding. The December 21 deadline to pass legislation funding the federal government, then, represents Trump and the GOP’s last, best shot at the wall — and the president is threatening to shut down the government if Congress does not send him a spending bill that appropriates $5 billion for the border wall.

That total is far short of the $20 billion which Trump has demanded for the wall in the past, but it’s a down payment that would give the president something to champion for his base as he heads into a daunting 2020 re-election campaign. If that falls short, he’d at least go down fighting: at an explosive Tuesday meeting with Capitol Hill’s top two Democrats, Chuck Schumer and Nancy Pelosi, the president said he’d be “proud” to shut down the government over the wall.

The border wall remains broadly popular with the president’s supporters, including those in Congress, but it’s anathema to Democrats and their party base. Congressional Democratic leaders have grown less willing to compromise on the wall with control of the House within their grasp, and Schumer and Pelosi are offering Trump no more than $1.6 billion for “border security” measures in the year-end bill, but they won’t budge on the wall.

The last-ditch effort to build the wall — and the president’s embrace of a shutdown in order to make it happen — is unfolding as key details about the project remain muddled, and as public opinion swings away from the president.

Where’s wall, though?

The border wall has been a consistent Trump theme since he kicked off his presidential bid in June 2015, but Trump has not been consistent on key details, like how long the wall would be, what it would look like and what it would cost.

After suggesting the whole, 2,000-mile border might be walled, Trump said he’d build a 1,000-mile wall along the southern border. Since, he has scaled the length of the barrier down to “700 or 900” miles. Beyond that, he has said the wall would cost $4 billion; he’s also said it could cost $20 billion, a figure reportedly in line with internal estimates from the Department of Homeland Security. Trump has said the wall could be 35 feet tall, or 45 feet tall, or higher, and he’s given several different ways Mexico could pay for it, all of which have been soundly rejected by Mexico.

What is known: the wall would cover some stretch of border between the U.S. and Mexico, and it could include constructing new barriers and also improving existing barriers. Fixing existing walls is something the administration has found money to do, fueling Trump claims that the wall is being built already. About 650 miles of the border — particularly around what once were high-volume crossing areas in California — already have some wall or fence, but only 300 miles of fencing is designed to stop the movement of people, according to a USA Today investigation. Many of the roughly 1,350 miles of unfenced border are in remote areas with unforgiving geographical features, which even Trump has said makes a wall unnecessary in those areas.

The administration commissioned the construction of eight different wall prototypes, put on display near San Diego, which Trump inspected in February. But none of the designs have been officially selected. (In an August report, the Congressional Budget Office found significant design and engineering flaws with each prototype. The federal government spent $5 million total on the prototypes.)

The lack of clarity on the wall’s length and design not only make it difficult to put a price tag on the project, but it’s also prevented experts from assessing how effective it would be in doing what it’s supposed to do: prevent undocumented immigrants from crossing into the U.S. and stopping the flow of illicit items, like drugs, across the border.

After years of decreasing illegal border crossings, apprehensions of undocumented immigrants by U.S. officials at the border have been relatively high in recent months: in October and November, over 100,000 migrants were apprehended by the Border Patrol. But CBP statistics do not specify how many of those migrants were caught in attempts to cross the border, and how many of them were seeking asylum.

The practice of asylum-seeking, which is legal, is the current focus of the U.S. immigration debate. That’s largely due to the headline-grabbing “caravan” of migrants fleeing violence in Central America, who made their way to the border to apply for asylum at official ports of entry or to cross the border and begin the asylum process on the U.S. side.

According to Ana Pottratz Acosta, an immigration attorney who teaches law at Mitchell Hamline School of Law, asylum seekers account for a large share of the current uptick in border apprehensions reported by CBP. She said that other factors, like seasonal weather conditions, play a role in the uptick, too.

Pottratz Acosta is doubtful that a wall, which she calls symbolic, would do much to stem illegal border crossings. “I think we’re at a point where the border may be as secure as it’s going to get.” She expressed concern that, if the wall project goes forward, it would divert badly-needed resources elsewhere in the immigration system — like for additional officers to process the long backlog of asylum claims.

In a 2017 report, the Department of Homeland Security echoed that sentiment: “Available data indicate that the southwest land border is more difficult to illegally cross today than ever before,” federal officials wrote.

Who’s saving face?

Inconsistent as he may be regarding basic facts about the wall, Trump has been consistent this year in his willingness to risk a shutdown in order to get funding for the project. As early as July, Trump threatened to shut down the government if Congress does not send him a spending bill with $5 billion in funds for it.

The midterm elections made that demand much more urgent: With Democrats taking power in the House on January 3, the spending legislation, which appropriates funds for five federal agencies, is the last piece of must-pass legislation for the GOP-controlled Congress.

It’s now or never for congressional Republicans on the wall, but unanimous support in the House and Senate GOP for funding it is hardly a sure thing. There may be enough members of the party’s moderate and conservative wings — who oppose the wall as bad policy and/or a waste of taxpayer money — to sink the proposal in either chamber, or both.

In a statement to MinnPost, Rep. Tom Emmer, the 6th District Republican, indicated he’d side with the president. “Border security is paramount to the safety of our community here in central Minnesota as well as across the country,” he said. “Unfortunately Congress has gotten so far away from completing the budget process on time that we now find ourselves in the same situation we were in January, when the Democrats shut the government down over a deadline that did not exist,” Emmer said, referencing the short-lived January shutdown over the status of the Deferred Action for Childhood Arrivals program.

“I am hopeful we avoid a repeat of that misstep.”

Emmer’s two House Republican colleagues from Minnesota, Reps. Jason Lewis and Erik Paulsen, were defeated in the November election. Neither responded to requests for comment about their position on wall funding, but in a November 28 tweet, Lewis accused Democrats of wanting “Schumer Shutdown 2.0” to stop the wall.

Republicans are outwardly expressing confidence they have enough votes within their own ranks to approve wall funding. To prove it, Republican leadership is considering putting stand-alone legislation on the wall to votes in the House and Senate, according to Politico.

But the broader GOP shakiness on the issue gives Democrats leverage to vote no on a package that includes more than they’d want to pay for the wall. Members of the soon-to-be-majority have been united in their opposition to the GOP proposal: “Real investments, smart investments in border security that are effective and humane have my support,” said 4th District Rep. Betty McCollum, a member of the House Appropriations Committee. “But wasting $5 billion on an unnecessary wall so President Trump can save face is foolish and not going to happen.”

“Republicans control Congress and the White House – they will either negotiate and legislate or shut down the government,” McCollum said in a statement. “This is a Republican mess of their own making.”

In the Senate, DFL Sen. Tina Smith expressed hope that Trump and Congress could come to an agreement and avoid a shutdown. “Border security is an essential component of our national security, and I believe that rather than funding an ineffective and wasteful wall, a spending package could include practical, effective security improvements like detection and surveillance technology,” she said in a statement.

Congressional observers have pointed out that Democrats could call Trump’s bluff, and endure a shutdown — which would only be partial, since seven out of 12 appropriations bills have already passed — until they take power in January. (About 25 percent of the government would not get funded in a shutdown, affecting the Department of Homeland Security, the State Department, the Department of Justice, the Department of Agriculture and the Department of the Interior, which operates national parks.)

Public polling on the wall suggests Americans may be more likely to place blame on Republicans if a shutdown happens. A new survey this week from PBS NewsHour/Marist College found that over two-thirds of Americans believe building the wall should not be a priority, and 57 percent want to see Trump compromise in order to avoid a shutdown. Two-thirds of Republicans polled, however, want to see Trump hold firm — even if it results in a shutdown.