From his 2016 campaign to his Oval Office remarks this week, President Donald Trump has returned to a border wall as the best solution for the U.S.-Mexico border.

In Tuesday’s primetime address, amid a three-week-and-counting government shutdown, Trump did point to other solutions. He cited a Homeland Security proposal that requested “cutting-edge technology for detecting drugs, weapons, illegal contraband,” as well as “more agents, immigration judges, and bed space” to process immigrants coming into the U.S.

But his $5.7 billion request for a border wall continues to stall negotiations over reopening the government, both after last night’s address and a Wednesday meeting at the White House. Today, the president also signaled again that he would consider declaring a “national emergency” on the border to help fund the wall, if he couldn’t reach a deal with Democrats.

“This barrier is absolutely critical to border security,” Trump said in his address Tuesday night.

It’s a refrain he has repeated many times and put even more succinctly in 2017 during a news conference with Colombia’s president. “Walls work,” Trump said.

Numerous security officials, and members of Trump’s own administration, have said a physical barrier should be, at most, only one part of a broader immigration strategy.

It’s an issue that divides Americans.

A Reuters/Ipsos poll released Tuesday found about 41 percent of the American public supports building more border fencing. That is down 12 percent from a similar poll in 2015, but support among Republicans remains high. About 77 percent of Republicans want more fencing and 54 percent support Trump shutting down the government until Democrats agree to fund the border wall.

Here’s a look at what experts say about the best options for border security, and how they stack up against the president’s and Democrats’ plans.

Does a wall work?

As a candidate and as president, Trump has wavered on what exactly a border wall should entail.

At times, he has insisted on 1,000 miles of solid concrete wall that stretches 35 feet high. Other times, he has floated the idea of a solar wall. More recently, he said he would consider using steel instead of concrete.

Trump administration officials have offered more options — putting less emphasis on a “wall” and suggesting that the barrier could actually be a mix of “steel slats” and “technological enhancements.”

Whatever its final form, Trump has held firm on the view that a physical wall is the key part of immigration enforcement. Most experts, however, disagree.

Physical barriers can be effective, mostly as deterrents against illegal border crossings or as a way to channel migrants to areas along the border where border patrol agents can more easily monitor them. But barriers alone don’t stop migrants from attempting to cross the border altogether, said Robert C. Bonner, who served as commissioner of the U.S. Customs and Border Protection under President George W. Bush.

“Physical barriers are not a strategy. They are part of a strategy,” Bonner said.

The terrain of the U.S.-Mexico border, which spans nearly 2,000 miles, is diverse. A fence might be the best deterrent along some parts of the border, Bonner said, while other areas would be more secure with sensors and an increase in border patrol agents.

WATCH: For a ‘smart’ secure border, ‘we need need some creativity,’ says Rep. McCaul

A 2014 report from Rep. Michael McCaul, R-Texas, then the chairman of the Homeland Security Committee, detailed how each part of the border needs its own unique set of fixes.

The Yuma sector of the border in Arizona, for example, includes 126 miles of mountains, deserts and large sand dunes, and already has 62 miles of pedestrian fencing. McCaul noted in the report that stretch of fence “effectively discourages large groups of immigrants from simply walking across the border and provides Border Patrol agents greater time to identify and respond to threats.”

“Physical barriers are not a strategy. They are part of a strategy.”

But the report did not suggest building barriers along other parts of the border. These regions include the Big Bend border sector in Texas, which encompasses a 510-mile section of border territory along the Rio Grande River that McCaul’s report characterized as a “low-threat area” for illegal immigration. There are only 4 miles of fence in the entire sector, despite the fact that it accounts for a quarter of the country’s U.S.-Mexico border. Yet McCaul did not recommend additional fencing for the area, instead calling for more drone surveillance and “improved communications capabilities.”

In the San Diego area, which has more than 40 miles of border fence, McCaul’s plan recommended increased tunnel detection capabilities for migrants and drug smugglers digging underneath the fence — a technology officials say still needs improvement, more aerial detection capabilities, and the expanded use of “game cameras” — devices typically used for fighting wildlifes that automatically take pictures when movement is detected.

The House Republican’s 2014 report is just one example of proposals from both sides of the aisle over the years that have stressed a multi-layered approach to the border.

Others have long argued that barriers, whether walls or fencing, have unintended consequences.

“The more complicated you make crossing the border, the more sophisticated the efforts to go around it or go underneath it become,” said Eric Olson, a senior consultant for the Wilson Center’s Mexico Institute. “The ones with the capacity to do that are organized crime.”

Rebuffed by fences, more immigrants in recent years are entrusting themselves to criminal enterprises or trying to cross on their own through treacherous terrain, including deserts. By some estimates, immigrant deaths along the border are five times higher than they were a decade ago. The International Organization for Migration also recorded more deaths in 2017 than a year earlier, despite a drop in the total number of people attempting to cross the border.

When choosing what material to use for border barriers, immigration and security experts also said a solid wall is not the most practical approach. It’s critical for Border Patrol agents to be able to see what’s happening on the other side of the border, to prevent them from getting caught off guard by people or vehicles trying to make their way across. Solid walls reduce that visibility.

David Bier, an immigration policy analyst of the libertarian Cato Institute, said it’s “fanciful thinking” if lawmakers expect a higher or longer fence to stop people from making unauthorized entries into the U.S.

“Even if you get a sea-to-shining-sea wall, then people would just build ladders, ramps and other ways — tunnels — in order to get around it,” Bier said. “It’s just not reflective of the reality, which is people will come if they want to come.”

Gil Kerlikowske, who served as the head of Customs and Border Protection under President Barack Obama, said the border wall is “kind of one more tool.”

A number of Democrats, including then-Senators Obama, Hillary Clinton and current Senate Minority Leader Chuck Schumer, voted in favor of the Secure Fence Act of 2006, which authorized building 700 miles of fence on the U.S.-Mexico border. Senate Majority Leader Mitch McConnell and other Republicans are now pointing to the votes as proof the Democrats’ opposition to Trump’s wall is hypocritical.

Kerlikowske noted that Bush and Obama built new stretches of fencing but also improved operations at the border with fence repairs, more personnel and more technology.

In contrast, under Trump, “I think the thing that’s so harmful in this whole discussions [around the border wall] is that we’ve made it this incredibly simplistic answer,” Kerlikowske said.

Costs of a “wall” are complicated

Customs and Border Protection’s annual budget is $13.9 billion.

Trump is demanding $5.7 billion to build a border wall. But the wall would not be a one-time cost.

In 2010, the Customs and Border Protection estimated it would cost $75 million to maintain the existing 650 miles of border fencing. Trump’s proposed wall across half of the 2,000-mile border would be nearly twice as long.

Technology upgrades also cost additional resources each year. Customs and Border Protection’s budget allocated nearly $35 million 2018 for information technology systems services and equipment used to detect and deter illegal border activity across the entire country, not only on the U.S.-Mexico border. Another $3 million was allocated for sensors, $24 million for communications systems, and $44 million for surveillance.

What are the alternatives?

Democrats are refusing to fund Trump’s border wall. Instead, the House of Representatives passed a bill Jan. 3 that would maintain the current level of border security funding — $1.3 billion — that could be used for new fencing and repairs of current barriers.

In response to Trump’s Oval Office address, Schumer and House Speaker Nancy Pelosi said they would support improved infrastructure and roads at ports of entry, new technology to scan cars and trucks for drugs and detect “unauthorized crossings,” and more personnel.

U.S. Customs and Border Patrol already uses a wide variety of tools to enforce border security, including drones, thermal imaging, radar and ground sensors.

If he were in charge of border security today — now that technology is more advanced than it was in the early 2000s — Bonner, the former CBP head, said he would want to make sure there were detection capabilities “for literally every yard of the border.”

That way, border patrol agents would know not only how many people they apprehend, but also how many people they did not catch, Bonner said — an added layer of information that would help the agency determine how effective its tactics are.

More attention could also be paid to the more than 40 legal ports of entry along the U.S.-Mexico border.

An increasing number of asylum seekers have been turned away at legal ports of entry because immigration enforcement cannot keep up with the number of claims. The total number of people seeking asylum increased nearly 70 percent from 2017 to 2018, according to Customs and Border Patrol data.

“The problem is the backlog that has built up over decades at the ports of entry for processing asylum claims, which is not solved or even addressed by a border wall” said Olson of the Wilson Institute.

Olson said if more resources were put toward processing asylum claims, asylum seekers would be more inclined to apply through legal ports of entry instead of crossing into the country illegally through other parts of the border.

Others ideas include cracking down on drug smuggling by prescreening more Americans through programs like TSA pre-check and companies through the Customs Trade Partnership Against Terrorism program. The programs are designed to speed up the flow of people and goods at ports of entry and improve border security by allowing agents to spend more time focusing on people and vehicles that could be carrying drugs or pose other risks.

Do the solutions address the ‘crisis’ at the border?

Trump has argued that a fortified barrier along the southwest border would help slow drug trafficking, terrorism and the number of unauthorized entries into the U.S. However, experts — and the government’s own statistics – -suggest the president’s characterization of the border “crisis” is misleading.

The number of apprehensions in recent years have been at historically low levels.

For example, while Trump lamented the amount of drugs entering the U.S. during his address Tuesday, the federal government has released its own data concluding that the “bulk” of drugs are transported through official ports of entry.

In his speech the president also painted a picture of unchecked immigration. But the number of apprehensions in recent years have been at historically low levels, especially compared with 2000 — widely considered the peak year for illegal immigration in recent U.S. history — when border officials flagged more than 1.6 million migrants for unauthorized entry into the U.S. For fiscal year 2018, that number was fewer than 400,000.

And in focusing on crime and drugs, Trump also did not address the underlying reason behind the rise of families and unaccompanied children — many from Central America — arriving at the border. Many immigrants are fleeing persecution, extreme poverty and violence.

A 2017 report from the Center for Migration Studies, based on 2014 data, also found that 42 percent of all undocumented people in the U.S. were “overstays,” or individuals who originally entered the U.S. with an approved visa, but then overstayed the visa limit and remained in the U.S.