Like others have said, if Trump ran on his 2016 platform 8 years earlier he would still be tapping into a very strong right wing voting bloc. He'd be a force to be reckoned with, but it's hard to say whether or not he'd win the GOP nod in 2008. He snatched it in OTL partly because the Republican establishment was divided between a multitude of several unappealing candidates, whereas 2008 had a much smaller crowd with (at the time) nationally popular candidates like Giuliani (who for a time lead Clinton in the polls) and McCain. I say he has a good shot to clinch the nomination, but at a time with more moderates in the GOP there could be a stronger movement within the party to unify to stop him as Cruz and Kasich (no moderates themselves) tried in 2016.
If Trump is nominated anyway, then voters would be left with an even more uncomfortable choice than 2016: vote for a shady real estate mogul and open bigot or hold your nose and vote for the Dems who oversaw the global economic meltdown. I could see Bloomberg launching an independent candidacy in this event, as he considered doing in 2016 but declined at the last minute to endorse Hillary.