I’d prefer a vote for Romney and a concurrent loss for Hillary who, BTW, probably has the nomination locked up no matter the votes in the Primaries (40% so-called ‘Super Delegates’)
A rash assumption on your part to assume Mitt would actually have a chance of getting 40% of the general election vote.
So here's the likely scenario should he get the nod: 50-60% of the GOP voters who were previously against the notion of a Romney presidency would go ahead & vote for Mitt. 40-50% won't.
Those who voted for Mitt would then have on their conscious the precedent that they voted for a RINO. And once you have on your track record that you voted for a RINO, you've already caved & compromised.