We are also deciding that question on the basis of a phrase ("reasonable expectation of privacy") that doesn't even appear in that 200 year old document.
Between gerrymandering, faulty or "faulty" voting machines, modern campaign practices that require millions of dollars to run and stick your "branding" to the minds of voters, and even more important, a system that pretty much guarantees no-one can rise to validly compete grass-roots unless they belong to the two parties -- it's nothing to write home about.
That a 200+ year old document is held as some kind of "holy scripture" of governance and that centuries old dead statesmen are seen in a romantic light as "founding fathers" and guiding spirits (paternalism much?), is even less indicating of a rational modern democracy.
This line of thinking is the source of lots of problems. The document has been amended 27 times. It's not infallible.
Never understood those US ideas on such matters. Nobody would want/expect/wont be furious to find out their two party communication was read by somebody else who wasn't meant to get it (and I don't mean show to some third person by your correspondent).
It's not gauging the public belief/sentiment on the matter, but BS ad-hoc "interpretations" of legacy laws allowed courts/pundits to even make such inane claims that people don't expect privacy for their mails.
If we're talking about expectations, then how about them asking people what they expect DIRECTLY (eg with a state/national poll) instead of some court deciding.