“A politician thinks of the next election; a statesman of the next generation” is credited to American preacher James Freeman Clarke (1810-1888), whose authorship was cited in 1870.
 
The full quotation (from a publication called Old and New) is rarely recorded in modern anthologies: “A politician thinks of the next election; a statesman of the next generation. A politician looks for the success of his party; a statesman for that of the country. The statesman wished to steer, while the politician was satisfied to drift.”
 
   
Wikipedia: James Freeman Clarke
James Freeman Clarke (April 4, 1810 – June 8, 1888), an American preacher and author.
         
3 February 1870, Cincinnati (OH) Daily Gazette, pg. 1:
James Freeman Clarke says: “A politician thinks of the next election; a statesman of the next generation. A politician looks for the success of his party; a statesman for that of the country. The statesman wished to steer, while the politician was satisfied to drift.”
 
18 February 1870, Jamestown (NY) Journal, pg. 1:
James Freeman Clarke says: “A politician thinks of the next election; a statesman of the next generation. A politician looks for the success of his party; a statesman for that of his country. The statesman wished to steer, while the politician was satisfied to drift.”
   
9 August 1883, Cambridge City (IN) Tribune, “Gems of Thought,” pg. 1, col. 6:
A politician thinks of the next election; a statesman of the next generation; a politician looks for the success of his party; a statesman for that of his country. The statesman wishes to steer, while the politician is satisfied to drift.
 
1 September 1883, Mountain Democrat (Placerville, CA), “Sayings of Sages,” pg. 4, col. 3:
A politician thinks of the next election; a statesman of the next generation; a politician looks for the success of his party; a statesman for that of his country. The statesman wishes to steer, while the politician is satisfied to drift.
 
Google Books
September 1888, Frank Leslie’s Sunday Magazine, pg. 234, col. 2:
THE American, of Philadelphia, says of James Freeman Clarke, the eminent Unitarian preacher who died recently of Boston: “He ought to be remembered for his saying, ‘The politician thinks of the next election; the statesman, of the next century.’”
 
Google Books
Wit and Humor of Well-Known Quotations
By Marshall Brown
Boston, MA: Small, Maynard & Co.
1905 [1904]
Pg. 230:
A politician thinks of the next election; a statesman of the next generation. A politician looks for the success of his party; a statesman, for that of the country. The statesman wishes to steer, while the politician is satisfied to drift.—James Freeman Clarke.