Interesting. On the one hand, Romney really shouldn't be able to pull a Grover Cleveland, while on the other hand, COVID-19 would destroy pretty much any incumbent's chances at reelection especially an incumbent as polarizing as Hillary Clinton.
Former President Mitt Romney (R-MA)/Senator Marco Rubio (R-FL) 49.888%
President Hillary Clinton (D-NY)/Vice President Tim Kaine (D-VA) 49.888%
P.S. I'd like to believe Trump is smart enough to not primary an incumbent President, OP. Or was he running third party, which makes less sense, since he was a registered Republican during the Obama Administration.