It's hard to shake the feeling that Trump's testing the waters...
It's worth noting that Trump isn't having people swear to vote for the Republican nominee, he's having people pledge their vote to him, Donald J. Trump.
I imagine everyone here is familiar with the "Fuhrer pledge" which made the German armed forces an extension of Hitler's will, but the very next day the German government also had all public officials swear to be "loyal and obedient" to Hitler, personally.
Trump's pushing things as far as he dares right now, but if he gains the presidency...I'm not sure what limits he'd be willing to recognize.
Edit: my bad, it wasn't the day after.