I'm not convinced that Romney would have carried NH, IA, or ME-2.
Romney lost in 2012 because he couldn't hide his elitism. That, and his crass insincerity. The only advantage of Trump that Romney would share was Hillary's negatives. It would have been an election with minimal voter enthusiasm.
Obama approvals though weren’t that low in his first term in the aggregates for him to lose . His lowest average approvals in his first term was in 2011 and it was -1.4 and the reason is while Obama approvals did have major drops they would always quickly rebound so I don’t think that election was ever winnable . Also keep in mind Hillary even in 2008 trailed McCain in the polls in Iowa while Obama was way ahead so Obama always was a great fit for Iowa .
Also the most damaging attacks on Romney wouldn’t work for Hillary as Clinton was the one who permanently normalized trade relations with China and let them into the WTO while Romney had been railing against Chinese trade practices since 2007. She also couldn’t attack him on Wall Street as she too raised a lot of money from there
https://www.realclearpolitics.com/epolls/other/president_obama_job_approval-1044.html