As far as I can tell, it's a matter of semantics. The Democrats were all in favor of 700 miles of fencing and technology upgrades during the end stages of the Bush administration and yet for some reason it wasn't completed during the Obama era. Now, trying to get it back on track and calling it a wall has got the same Democrats up in arms about the possibility that Trump may fulfill a campaign promise and a self-serving media is keeping the short attention span folks up in arms about all those poor unfortunates who simply must enter the country illegally because we're all such racists (that's really good for ratings ya know).
I think it's a bad idea to declare a national emergency to accomplish the goal, but if the political landscape accepts the Democrats efforts to dis-approve what they've previously approved, the same landscape will allow the President a little leeway in accomplishing it.