So by your logic
The republicans should have stood aside and not complained when the democrats passed Obamacare, since that was one of his signature campaign promises? Being elected as president doesn't mean the electorate automatically supports all your campaign pledges - especially when the margin of victory was less than 100K votes across three states. Even Obama's somewhat larger margins didn't grant him a "mandate" to do anything he promised as far as I'm concerned. The congress is an independent branch of government, not a rubber stamp for the president's wishes like Trump thinks they should be.
And as suggested, if it was this important to Trump, why didn't he push it harder when he had both the house and senate? It is pretty obvious he's only doing this because a couple conservative media figures loudly objected, and he panicked about losing support from his base. If he believes he's able to declare a state of emergency to get his way, and ends up doing so, then what he's done by using the federal employees as pawns is abhorrent. He'll probably do so, only because it is the only way out for him at this point without losing face.