First of all, Trump ran under the republican party since the Dems had already decided that Hillary would be their candidate. Everyone knew this two years ago. The Dems were certainly not expecting such a millenial surge of support for Saint Bernie of Commeicut.
Second, since Trump was not "loyal" to the Dems, why should he be loyal to the Republicans? The GOP did not want Trump in any way, shape or fashion until it was obvious that the Trump Train was picking up steam. Unlike the Dems, the Republicans saw that this would be their ride to the White House.
It can be debated that the Republicans figured they could control Trump once he got to the White House, even though they wanted - and still don't want - nothing to do with him.
Trump is lambasted in the MSM hourly. More so than any other POTUS had to face. An already difficult job compounded by having to defend himself multitudes of times over and over. And you wonder why he turns to Twitter. Well, that's to speak directly to the people who elected him - and those still interested in putting the country first instead of party affiliation. Which fuels the MSM outrage for circumventing their ability to shape his message to the unwashed masses to how they see fit.
oh - there's already legislation put forth to fund the wall. let's see which ******** party blocks or delays it.