A politician is called a “flip-flopper” when he or she “flip-flops” (changes position) on an issue. The terms come from the 19th century; an older term is “somersault.”
 
“Flip-flop” has been cited in print since at least 1876 and “flip-flopper” has been cited since 1878.
 
   
Wikipedia: Flip-flop (politics)
A “flip-flop” (used mostly in the United States), U-turn (used in the United Kingdom and the Republic of Ireland), or backflip (used in Australia and New Zealand) is a sudden real or apparent change of policy or opinion by a public official, sometimes while trying to claim that both positions are consistent with each other. Often it will occur during the period prior to or following an election in order to maximize the candidate’s popularity.
 
Although the terms are often used against elected officials, non-elected public officials can also be accused of flip-flopping. Ben Bernanke, chairman of the U.S. Federal Reserve Board, was accused of “an apparent flip-flop” in 2007. Lawyers sometimes accuse their opponents of a flip-flopping, too.
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History
In his “On Language” column in The New York Times, William Safire wrote in 1988 that “flip-flop” has a long history as a synonym for “somersault”. (He cited George Lorimer in 1902: “... when a fellow’s turning flip-flops up among the clouds, he’s naturally going to have the farmers gaping at him.”) In the late 19th century, a U.S. politician was called “the Florida flopper” by an opponent, Safire noted. The “fl” sound appearing twice is an indication of ridicule, he wrote. Citing grammarian Randolph Quirk, Safire pointed out that the doubling of the sound is also a feature in other two-word phrases used to disparage the actions or words of others, including “mumbo jumbo”, “wishy-washy”, and higgledy-piggledy”.
 
In the archives of The New York Times, which go back to 1851, the earliest unequivocal mention of “flip-flop” as a change in someone’s opinion, is in an October 23, 1890, report of a campaign speech in New York City. John W. Goff, candidate for district attorney, said of one of his opponents: “I would like to hear Mr. Nicoll explain his great flip-flop, for three years ago, you know, as the Republican candidate for District Attorney, he bitterly denounced Tammany as a party run by bosses and in the interest of bossism. [...] Nicoll, who three years ago was denouncing Tammany, is its candidate to-day.”
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The term also was used extensively in the 2004 U.S. presidential election campaign. It was used by critics as a catch-phrase attack on John Kerry, claiming he was “flip-flopping” his stance on several issues, including the ongoing war in Iraq.
 
(Oxford English Dictionary)
flip-flop, n. (and adv.)
orig. U.S. Polit. A change of mind or position on something; a reversal.
1890 Chicago Tribune 13 July 6/5   Mr. Ericksen’s friends in the twenty-third executed a flip-flop, and‥went over to Michael Francis in a body.
1917 Fort Wayne (Indiana) Daily News 9 June 14/4   Cantrill executed an alert flip-flop today on the suffrage issue.
1940 D. L. Cohn Good Old Days Introd. p. xviii,  Sears does a flip-flop and goes in for time-payment merchandising.
 
(Oxford English Dictionary)
flip-flopper, n.
A person, esp. a politician, who (habitually) changes his or her opinion or position.
1894 Chicago Daily Tribune 27 Mar. 8/5   That incomparable political flip-flopper‥was rewarded for his last flop with a fat diplomatic position.
1915 Los Angeles Times 24 Jan. 8/3   [He is] running President Whiffen a close race as the Council ‘flip-flopper’. He has changed his mind again on the subject of the salaries ordinance.
 
4 September 1876, Decatur (IL) Daily Republican, pg. 2, col. 5:
The Chicago Times is unsparing of its enconiums upon “flip flop” candidates for congress, as witness: “J. C. Black has been nominated for Congress in the fourteenth district by the Illinois political ornithor hynchus. He is a natural born political double ender, as exhibited in a telling (?) speech in favor of ‘Tilden and reform’ and against specie resumption, which he said he would abolish the people’s money.”
 
Chronicling America
16 February 1878, The People’s Vindicator (Natchitoches, LA), pg. 2, col. 3:
Col. Wise, of Caddo, we see, signs the majority report for the amendments. The colonel, we hope, is not an arenic Artist—that is a flip-flopper, in statesmanship, but the wires last week made the Shreveport Times say, he would sign the minority report, and favor a Convention in a speech. It’s a minor mistake of major importance, however.
 
Chronicling America
21 June 1878, Brenham (TX)

, pg. 1, col. 2:
The Austin Gazette has completed its flip-flop.
   
3 May 1879, Salt Lake Tribune (Salt Lake City, UT), pg. 1:
CONSISTENCY.
Somethings That Can Flip-Flop Afterward Better Than Forward.
Gov. Emery, as early as the 20th of March last, was earnestly requested to call the May term of the Second District Court at Silver Reef.
   
Chronicling America
12 October 1880, St. Paul (MN) Daily Globe, pg. 2, col. 5:
Loose and Tight.
[Waseca Radical.]
Harding’s Herald the righteous flip-flopper, puts this conumdrum: “Why is it so hard to get Ward out of canvass?”
 
19 March 1881, Omaha (NE) Herald, pg. 1:
FLIP FLOP.
Mahone Flips Over and Votes
With the Democrats.
 
20 September 1888, Evening Repository (Canton and Massillon, OH), pg. 2, col. 4:
Will He Flop Again?
It is suggested that President Cleveland, who has reversed his fishery policy, may turn another somersault and “go back on” the Mills bill, his own pet scheme. Perhaps so. He has turned his back on about every other position he has held, including his anti-second term attitude. In fact, he is making a record as the great American flip-flopper. But he can’t wipe out his free trade record, whatever contortions he may make.—Troy Times.