It would be a dysfunctional Senate if everyone acted as if everyone else was lying all of the time. A Senator 's promise was currency. The deals were "vote for my bill X today and I'll vote for your bill Y next month." If you reneg next month, nobody will trust you again and you won't get anymore bills passed.
Now everyone knows that McConnell is a dishonest negotiator, at least with fellow Republicans. He probably keeps all of his promises with Democrats.
-PJ
All he needed to do was have a discussion with Jim Bunning.