A few commenters at this last post on presidential campaign spending suggested examining spending as a function of annual real GDP. Check it out:
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.
Another observation: 2008 looks pretty unremarkable.