The “Dems are responsible for Trump because they rejected Romney” line is predicated on the illusion that Trump created the modern GOP rather than the other way around. In 2012 Romney was playing with the same forces as Trump. https://www.theatlantic.com/ideas/archive/2020/05/birtherism-and-trump/610978/
The different incarnations of Romney—moderate Mass gov, birther-curious GOP nominee, Trump skeptical Utah Senator—are reflections of the constituency he was serving in each instance. The GOP that produced Trump already existed in 2012, which is why Romney sought his endorsement.
Romney would have made a more competent president than Trump, but the idea that he was impervious to the ideological currents that led to Trump, or that Trump was a break with American conservatism rather than an expression of it, does not stand up to scrutiny.
I understand why people would like to make Trump and Trumpism issues of personal virtue. There’s a political appeal to it, and it makes things less scary. But the truth is that Trump is an expression of the constituency he serves, which is why GOP lawmakers can’t break with him.
Liberals don’t actually have the power to make conservatives want things they don’t want. Trump is there because conservatives want him there.