Mitt Romney, presidential bid as a democrat?

This. The fact that he’s an anti-Trump Republican may give him a modest level of respect among some Democrats, but compared to any actual Democratic candidate, his views and policies are going to be far too conservative. He may be marginally not as unpalatable to liberal voters as Trump or his MAGA allies, but that doesn’t mean he’s actually palatable to them.

See a not-dissimilar discussion here, from last year, about Liz Cheney: Liz Cheney for president. Who would vote for her?

Also, memories aren’t that short, and liberal voters will remember his comments from his 2012 presidential run, like: