Both sides do it.
In this case, I saw a screen shot of a document which showed Clinton writing to Blair asking his help in solving a situation to help the Democrats in the election.
-- Bill Clinton: "... In a political season, it would be big over here to get this open sore resolved. If you could have somebody take a look at it."
-- Tony Blair: "Sure, I will have a look."
I can see someone characterizing that, and saying no quid pro quo, or whatever, but the proof is the same as it is for Trump. Granted, the Trump supporters will characterize what he said, just like the anti-Trumpers will characterize what Clinton said, or just discount it as irrelevant because it is "whataboutism." But to say that no proof is ever offered is silly. It is there, although it is sometimes not reported by all news outlets, depending on their agenda.