He wouldn’t have governed as president like he did as governor of Massachusetts.
And keep in mind that when you elect a president, you’re doing more than just putting one guy in office. He brings a staff and a gazillion appointees, many of whom will be conservatives. Basically it’s a wholesale handover from the Democrat establishment to the Republican establishment. The president is the most visible figure but it’s a vast change that goes deeper than him. Lots of these appointments would get filled by the same people regardless of who the particular Republican president is. When you sit home out of spite because your preferred candidate got beat in the primary, you’re also helping keep these people from taking positions in government. You’re choosing instead to leave a whole unseen locust swarm of liberals in place.
I’m sorry, but that’s delusional. Romney started out as a liberal, lied when he wanted to win the GOP presidential nomination, and then began immediately to revert once that election was over.
Had he won, he would have governed as a liberal. But he would have done it behind a Republican mask, in YOUR name.