The most expensive election ever
Seth Masket says it was 1896:
What this chart mainly demonstrates is what an enormous outlier 1896 was. 0.06% of GDP went toward campaign spending. Another way to say that is that for every $1,000 spent in the U.S. in 1896, 60 cents went to finance a presidential campaign. And as we can see, that figure dwarfed those of other years.
My understanding is that spending in 1896 was largely driven by the Republican side, which outspent Democrats roughly 5 to 1. And that spending mainly came from large corporations enlisted by Marcus Hanna to beat back the silver-coining advocates they (correctly) perceived as a threat to their interests.