To “preach to the converted” means to bring a message before people who already believe (or are predisposed to believe) that message. The preaching doesn’t win over any new converts. although it may prevent the converted from leaving. The London Times printed in 1857, “It is an old saying that to preach to the converted is a useless office, and I may add that to preach to the unconvertible is a thankless office.”
 
“Preaching to the choir” is an American saying, cited in print since at least 1968 and apparently popular at that time in aviation. The choir attends church (or a similar institution) regularly and doesn’t need to be converted. A political candidate who “preaches to the choir” misses out on bringing the message to independent voters and to those of a different political party.
 
 
Wiktionary: preach to the choir
Verb
to preach to the choir
1.(idiomatic) Speaking as if to convince a person or group of something which that person or group already believes.
Usage notes
Often used to imply that a speaker is addressing the wrong audience or is deliberately addressing a compliant, non-challenging audience.
Synonyms
preach to the converted (less common)
 
The Free Dictionary
preach to the choir and preach to the converted
Fig. to make one’s case primarily to one’s supporters; to make one’s case only to those people who are present or who are already friendly to the issues.
   
wiseGEEK
What is “Preaching to the Choir”?
Preaching to the choir is an English idiom that means a person is trying to convince or persuade another person or group to believe in or agree with something that they already believe in or agree with. Preaching to the choir, preaching to the chorus, and preaching to the converted are all similar idioms with the same meaning, but preaching to the choir is the most commonly used.
     
(Oxford English Dictionary)
to preach to the converted and variants: to advocate something to people who already share one’s convictions about its merits or importance. Also (orig. and chiefly U.S.) to preach to the choir.
1857 Times 6 Nov. 7/4   It is an old saying that to preach to the converted is a useless office, and I may add that to preach to the unconvertible is a thankless office.
1867 J. S. Mill Exam. Hamilton’s Philos. (ed. 3) xiv. 319   Dr. M’Cosh is preaching not only to a person already converted, but to an actual missionary of the same doctrine.
1916 G. Saintsbury Peace of Augustans iii. 144   One may be said to be preaching to the converted and kicking at open doors in praising‥the four great novelists of the eighteenth century.
1970 Washington Post 24 Sept. a27/2   Foster spoke yesterday before a packed Air Force Association seminar‥. Admitting that this was like ‘preaching to the choir’, he nevertheless went on to detail a rather gloomy view of declining U.S. defense capabilities.
     
Google Books
Michigan Aviation
Michigan Aeronautics Commission, Michigan. Bureau of Aeronautics
Volumes 1-5
1968 (Google Books date might be incorrect—ed.)
Pg. 22:
We spend our time preaching to the choir, just as I am now. I’m talking to people who love aviation and have a vested interest. My time would be more productively spent talking to the Chamber of Commerce downtown, convincing them that an adequate airport is this community’s tie to the twentieth century; and if they don’t have an airport they’re like the community that didn’t have a depot fifty years ago.
     
Google Books
Hearings on military posture and an act (S. 3293) to authorize appropriations during the fiscal year 1969 for procurement of aircraft, missiles, naval vessels, and tracked combat vehicles, research, development test, and evaluation for the Armed Forces and to prescribe the authorized personnel strength of the selected reserve of each reserve component of the Armed Forces, and for other purposes. Ninetieth Congress, second session.
By United States. Congress. House. Committee on Armed Services.
Washington, DC: U.S. Government Printing Office
1968
Pg. 9357:
Admiral CONNOLLY. Mr. Hardy, in a way, if I may not be disrespectful, it is almost as though you are preaching to the choir.
Mr. HARDY. It is almost— what is that ?
Admiral CONNOLLY. It is almost as though you are preaching to the choir.  Because we really believe that we are going to be able to bring forth a fine design and go to work and build a fine airplane, but that is our experience.
 
24 September 1970, Washington (DC) Post, “Pentagon Expert Says Soviet Has 300 SS-9s” by Michael Getler, pg. A27:
Porter spoke yesterday before a packed Air Force Association seminar on “the threat” to national security. Admitting that this was like “preaching to the choir,” he nevertheless went on to detail a rather gloomy Pentagon view of declining U.S. defense capabilities in the face of an increasingly well armed Soviet Union.
   
Google Books
Tax Revision in an Inflationary Era:
Proceedings of Tax Foundation’s 26th national conference
By Tax Foundation
New York, NY: Tax Foundation
1975
Pg. 31:
And you’re quite right, the people tend to come and talk to the people who agree with them, and have the feeling somehow that they’re doing their job if they come around and preach to the choir.
 
Google Books
Safire’s Political Dictionary
By William Safire
New York, NY: Oxford University Press
2008
Pg. 513:
It (“on the same page”—ed.) is synonymous with sing from the same hymn book< (also

sing from the same song or tune, or from the same hymn sheet) and is related to preaching to the choir. The preaching and singing phrases have been dated to the mid-nineteenth century, but probably were around for many years before being committed to writing. The London Times noted in 1857 that “It is an old saying that to preach to the converted is a useless office, and I might add that to preach to the unconverted is a thankless office.” A hundred and fifty years later, The Washington Times headlined an editorial about former vice president Al Gore’s scheduled testimony to House and Senate panels run by “climate-panicked Demcorats”: “Al Gore to preach to the choir.”