Here's what will need to happen before Trump can build his Mexico border wall

President Trump will sign several immigration-focused executive orders at the Department of Homeland Security (DHS) on Wednesday, one of them a down payment on his prominent campaign pledge to build a wall between Mexico and the U.S. He had vowed that Mexico would pay for the wall, but since it has refused, he modified that to Mexico will reimburse U.S. taxpayers. Trump plans to shift already allocated DHS funds to start building the wall, probably using authority under the 2006 Secure Fence Act, which allowed the president to build 700 miles of fence; former Presidents George W. Bush and Barack Obama built 652 miles.

Trump needs money and authorization from Congress to build the remaining 1,200 miles along the border, and while Republicans in Congress have signaled their willingness to pony up, a lot will depend on cost. Trump told MSNBC last year he could build his wall for $8 billion, but most estimates put the cost much higher — Border Patrol officials said that the first 652 miles cost $2.3 billion, and not all of that is wall, and the Government Accountability Office puts the price tag for the rest at $6.5 million per mile of single-layer fence plus $4.2 million more per mile for more fencing and roads, not counting maintenance. A study in July from Bernstein Research put the total cost at $15 billion to $25 billion.

Additionally, the new fencing can't disrupt the flow of the Rio Grande and other border rivers under a 1970 treaty with Mexico, limiting building options, and much of the unwalled border is on difficult terrain or through land owned by ranchers who don't want a wall. Trump's other executive actions on Wednesday are expected to include hiring more U.S. Border Patrol agents and committing to jail more immigrants caught crossing the border, both of which would drain the DHS budget even more.

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Then there's lack of public appetitive for building an expensive wall. A late-November Quinnipiac University poll found that 55 percent of voters oppose building the border wall, while a Pew poll in August found that 61 percent oppose walling off the entire border. A 49 percent plurality of Americans told Reuters/Ipsos in late October that building a Mexico border wall would be a "waste of money," versus 31 percent who called it an "effective barrier"; in Arizona, ground zero of the anti-immigration movement, 47 percent picked "waste of money" and 34 percent chose "effective barrier."