If Trump Declares An Emergency To Build The Wall, Congress Can Block Him.
. Trump can win the Senate, but not the House. From cnn.com: "And even if (he) got funding, that's the easy part. The hard part is overcoming the myriad of lawsuits that would almost certainly ensue. The wall have to pass a number of federal environmental reviews -- a process that usually takes years and gets tied up in courts.
The wall would also have to be constructed through private property -- especially in Texas -- and those cases would have to be settled in court. Then there's Native American tribal land along the border. Either a deal would have to be cut with the tribes, or the wall would have to cut through a different section of Arizona instead to go around the sovereign land.
So many hurdles exist.