One of these days, the ploy will fail and the markets will tank despite all efforts to prop it up. Fear of imminent interest rate hikes is one potential trigger for such an event, but I don't know if there is enough fear for that to happen just yet. I do think that the outlandish valuations in the current market make the markets vulnerable to immunity to the manipulation and jawboning that has worked so well thus far, so traders and investors (especially the latter) should be on guard for a seminal change in sentiment that would presage a sharp downside move.
The conventional wisdom is almost universal that the markets will hold until the elections, but I have very serious doubts about things holding together that long.