“We campaign in poetry, but when we’re elected we’re forced to govern in prose” said then-New York governor Mario M. Cuomo in a speech at Yale University (New Haven, CT) on February 15, 1985. Cuomo often repeated: “You campaign in poetry, you govern in prose.”
 
The quotation has been often used in politics, especially during the successful 2008 presidential campaign of Barack Obama, who was known for his campaign speeches.
 
     
Wikiquote: Mario Cuomo
Mario Matthew Cuomo (/ˈkwoʊmoʊ/; June 15, 1932 – January 1, 2015) was an American politician and member of the Democratic Party. He served as the 52nd Governor of New York for three terms, from 1983 to 1994, Lieutenant Governor of New York from 1979 to 1982; and Secretary of State of New York from 1975 to 1978.
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You campaign in poetry. You govern in prose.
The New Republic (April 4, 1985)
 
The Yale Book of Quotations
Edited by Fred R. Shapiro
New Haven, CT: Yale University Press
2006
Pg. 182:
Mario Cuomo
U.S. politician, 1932-
“We campaign in poetry, but when we’re elected we’re forced to govern in prose.”
Speech at Yale University, New Haven, Conn., 15 Feb. 1985
 
Google Books
The Quote Verifier: who said what, where, and when
By Ralph Keyes
New York, NY: Macmillan
2006
Pp. 21-22:
“You CAMPAIGN in poetry, you govern in prose.” This political adage is most often attributed to former New York governor Mario Cuomo. Beginning in the early 1980s, Cuomo used ti often, in speeches, conversation, and writing. The saying has also been credited to Richard Nixon, though far less often than to Cuomo. (A journalist who attributed the line to Nixon later noted that in the 1000 campaign “George Bush campaigned in nursery rhymes.”) In an episode of NBC’s West Wing, President Jed Bartlet’s chief of staff used the poetry-prose line without attribution. Wherever it originated, this saying owes an unacknowledged debt to an observation credited to British writer Beverley Nichols (1898-1983): “Marriage is a book in which the first chapter is written in poetry and the remaining chapters in prose.”
Verdict: Credit Mario Cuomo, with a nod to Beverley Nichols for source material.
 
20 February 1985, Philadelphia (PA) Inquirer, “Soundings Eyeing ‘88: Politicians see how the land lies,” pg. A12:
“We campaign in poetry,” Cuomo has said. “But when we’re elected, we’re forced to govern in prose.” 
 
7 April 1985, Elyria (OH) Chronicle-Telegram, pg. F2, col. 4:
“You campaign in poetry, you govern in prose,” is his (New York governor Mario Cuomo’s—ed.) way of putting it.
   
Google Books
Public papers of Governor Mario M. Cuomo, 1987
By New York (State). Governor (1983- : Cuomo), Mario Matthew Cuomo, Governor (1983-1994 : Cuomo, New York (State)
Published by State of New York, 1990
Pg. 584:
We often campaign in poetry, but then we’re always required to govern in prose.
   
25 October 1988, Waterloo (Iowa) Courier, “Celebrity Cipher,” pg. D6, col. 1:
PREVIOUS SOLUTION: “You campaign in poetry. You govern in prose.”—Mario Cuomo.