I think Harris and Haley are two of the strongest contenders from their respective parties for the next couple presidential elections. I think that in a totally neutral political climate, it would be extremely close, maybe with an edge to Haley. I think Haley would be somewhat more popular among several minority groups than your average Republican candidate, and this would help reduce Harris' effectiveness, as I think part of Harris' strength would be her ability to make NC, GA, and AZ competitive through energizing minority turnout. Haley in general would do slightly better in urban areas than Republicans in '08/'12/'16, but Harris would do slightly better among white working class Midwesterners than Hillary did - so the Midwest would be extremely, extremely competitive.
So, I think it would entirely depend on who the sitting president is and how popular they are.
If Trump wins re-election in 2020 and maintains his low popularity through the campaign (or, for that matter, if Pence is now sitting president but also is somewhat unpopular), I see Harris coming out on top, albeit by winning several crucial states by tiny margins. In recent history, it's hard for a candidate to win a presidential election if the sitting president has been there for 8 years and is from their party - this kind of constant turnover is a force that I think is glossed over but certainly enough to swing an election. WI, PA, NC, NH, and FL are the closest and most important states over the course of the campaign, but the aforementioned political climate weighs down Haley enough for Harris to win by >2% margins in all five.
Sen. Kamala Harris (D-CA) - 322 EV (49.8%)Gov. Nikki Haley (R-SC) - 216 EV (47.3%)If Trump loses re-election in 2020, meaning that either Harris or some other Democrat is the sitting president, Haley likely edges Harris out, unless the sitting president is extremely popular. Haley also would likely edge Harris out if Trump is still president and somehow becomes significantly more popular leading up to the election. Once again, WI/PA/NC/FL/NH are very close, but this time Haley wins them, and CO/NV/MI/MN are made very competitive.
Gov. Nikki Haley (R-SC) - 294 EV (49.1%)Sen. Kamala Harris (D-CA) - 244 EV (48.2%)
