WASHINGTON — Standing in the way of a resolution to the government shutdown is a wall. A still mostly hypothetical wall along large sections of the southern border intended to keep immigrants from entering the United States illegally. A wall that President Trump promised his supporters he would deliver once he was in the Oval Office. A wall that if not built, some conservatives say, could cost Mr. Trump his presidency.
It is a big wall. Here’s what you need to know about it.
Building a wall along the southern border was one of Mr. Trump’s most repeated campaign promises.
As he described it during the campaign, the wall would not be one continuous barrier along the entire stretch of more than 1,900 miles of land between the United States and Mexico, but rather sections of a wall. Natural barriers and “Border Patrol people” stationed in certain parts of the Southwest would provide security in between.
And building the wall would be easy. “I’m a builder,” Mr. Trump said. The construction would not be as complicated as building a 95-story high-rise. “And I would have Mexico pay for it,” he said. “Believe me.”
The estimated cost of the wall would be $4 billion to $8 billion. It would be 35 feet to 45 feet high, with a “big, fat beautiful door right in the middle,” he said, which would be for people to enter the country legally.
Looking to 2020: Nearly two years into Mr. Trump’s presidency, there is no wall.
Mexico has refused to pay for it, and Mr. Trump’s supporters still want it.
“Build a wall. Build a wall. Build a wall,” supporters chanted when Mr. Trump spoke at rallies around the country in the lead-up to the midterm elections.
In October 2017, prototypes for Mr. Trump’s wall were unveiled, a the administration has issued two contracts to begin construction on parts of a wall in Texas. But construction on new portions of the wall has not yet begun.
When the Senate passed a government funding proposal this past week that did not include money to build the wall, conservative commentators and members of the House Freedom Caucus warned Mr. Trump that if he did not veto it and fight for the funding, he would lose his base, and with that, any chance of re-election in 2020.
“One way or the other,” Mr. Trump said on Friday, “we are going to get a wall, we’re going to get a barrier.”
Division over a border wall and the potentially dire political consequences are not a predicament unique to the Trump administration. In 2006, Congress funded the construction of 700 miles of a border wall and fencing, despite the reluctance of the George W. Bush White House and Senate Republicans, who had hoped for a more comprehensive overhaul of immigration laws.
At the time, House Republicans insisted on funding a wall because they said it was what their voters wanted in order to stem illegal immigration.
The terrain along the southern border is a mix of federal and private land.
Since 2006, the United States government has installed barriers — some meant to stop vehicles, some to stop pedestrians — and a “virtual” fence that turned out to be harder to build than what officials had thought. The virtual portion of the border wall was canceled in 2011, during the Obama administration.
Today, the nearly 700 miles of border wall made out of materials such as wire-mesh and chain-link fences, is still in place and in different states of disrepair. Funding has been made available to fix portions of the wall, and the Trump administration has been moving forward with replacing some of the old barriers with new ones.
The current wall snakes through private property and has created barriers that have been detrimental to wildlife. Photojournalists from The New York Times visited sections of the border wall in four states this year and described what they saw.
In the Rio Grande Valley, there was a towering structure made of steel or concrete. Along one stretch, there was thornbush that appeared impenetrable. In other areas, there are Border Patrol agents driving A.T.V.s and riding horses that were following migrant-sniffing dogs. In El Paso, a tall, steel barrier of fence panels topped with a solid steel sheet divided the desert.
Mr. Trump is demanding that $5 billion for the wall be included in a spending bill necessary to keep the government running.
Democrats have long been opposed to this, leaving Senate Republicans without enough votes to get the spending legislation passed. Democrats say a border wall is ineffective, in part because it constantly needs to be repaired.
The Trump administration says there is a “crisis at the border,” despite government data that shows illegal border crossings into the United States from Mexico are declining. In 2000, federal agents apprehended 71,000 to 220,000 people trying to illegally enter the United States each month. This year, the apprehensions range from 20,000 to 40,000 people each month.