2016: Vice President Obama vs. Trump

[President Johnson]

Obama has a lot of charisma, no baggage and is a great campaigner. He would have beaten Trump, considering the latter just won by 77,000 votes against Hillary. Black and young turnout would have delivered him the election. Obama keeps the Rust Belt and wins Florida and North Carolina on top. Trump takes Iowa and Ohio by smaller margins and keeps Arizona, which I believe wasn't ready to flip in 2016 unless there is an absolute landslide. Georgia would have been close as well. There is a significantly smaller share of third party votes.

✓ Vice President Barack H. Obama (D-IL)/Senator Timothy M. Kaine (D-VA): 322 EV. (51.23%)
Entertainer Donald J. Trump (R-NY)/Governor Michael R. Pence (R-IN): 216 EV. (46.17%)

[President Johnson]

Assuming President Clinton had the approval numbers that Obama had in real life, it'd be a cakewalk for Obama, whose charisma & likability would probably carry him over the finish line anyway.

Yeah, though some of Obama's high approvals throughout 2016 may have reflected on voters dislike for both Clinton and Trump (and the comparison with Obama).