You campaign in poetry. You govern in prose.

Mario Cuomo (1932-2015) American politician

In Fred Barnes, “Meet Mario the Moderate,” New Republic (8 Apr 1985)

This looks to be the first printed source for this maxim of Cuomo's, though he had been using it for a while.  An earlier version: "We campaign in poetry, but when we're elected we're forced to govern in prose." Sometimes attributed to Nixon.  See here for more discussion.