What evidence do we have that the Framers intended for Senate to "keep an eye on the long term interests of the nation"?

The fact that their terms are 6-years and that it’s a smaller group. The House of Representatives are elected every 2 years and are meant to track short term passions of the people. Also, originally senators weren’t directly elected by the people which further insulated them from the “passions of the mob.”

Other than the Framers had them appointed, and not elected, none whatsoever. They all recognized that established governments decay into tyranny. Jefferson imagined that every generation or so would have to fight their own revolution. Most of them would not have imagined a run of 250 years.

@gorillapaws and @josie both wonderful answers!

The evidence is even in the name. “Senate” is a term originating in Rome, and it roughly means something akin to a council of elders, with the implication of offering wisdom and guidance. It’s where concepts like advice and consent originated, which were later adopted by the U.S. Constitution.

(A lot of the lingo from the Roman Republic is found in the U.S. government. The seat of the Roman Republic was the Capitoline Hill in Rome – the origin of U.S. Capitol Hill.)

@bolwerk that reminds me, the age requirements also point to a less-passionate group. You’re required to be 30 to run for senate, but only 25 to be in the House, so they really didn’t want any passionate young whipper-snappers changing everything willy-nilly.

Except in cases where the rationale was stated explicitly or was reported by parties present at the time, the motives are at best inferences based on political theories common at the time.

Explicitly within the Constitution? Not a whole lot, to be honest. All of the above answers hit the nail right on the head. But I would argue that it is perhaps implied, especially if one considers the Constitution within the context of the Federalist Papers. I highly recommend reading some of them (you can do so online) if you are interested in the topic, they are very informative and, in my opinion, quite fascinating to read.