Except that totally dismisses all the other factors. How bad did the Fox Moderators/Rubio gutter attacks hurt Trump in the last debate?
I would suggest that, and not Romney, is what is hurting Trump currently.
I believe it is because the field is narrowing down. People who might have voted for Fiorina, Carson or the others are now voting for the non-Trump candidates.
I like his promise of a wall. Unfortunately, he really cannot articulate much of anything else. You can only get so far slinging insults and discussing your penis size. Eventually people realize that they're choosing a president.