Green’s Dictionary of Slang

(US) ‘one who schemes to procure corrupt votes’ (OED).

1840		Niles Wkly Reg. 28 Nov. 205/3: Corruption of the franchise by pipe layers and yarn spinners — aided by the most shameless humbugs.
1842		N.Y. Daily Trib. 16 Apr. 1/1: [headline] Who are the Pipe-Layers?
1848		Bartlett Dict. Americanisms 251: pipe-laying. This term, in political parlance, means any arrangement by which a party makes sure of a certain addition to its legitimate strength in the hour of trial—that is, the election. In other words, to lay pipe means to bring up voters not legally qualified.
1865		G.A. Sala My Diary in America I 421: ‘Pipe-laying’ means simply the secret management by intrigue and chicane, of a certain affair.
1872		Schele De Vere Americanisms 262: The term pipe-laying [...] is still used to designate the employment of persons as voters, who are not entitled to vote, by fraudulent means.
1897		C.H. Haswell Reminiscences of City of N.Y. 379: The Democratic papers dwelt upon the act and termed the perpetrators ‘pipe-layers’; which term [...] is still in current use to denote concealed and indirect methods of political or other actions.
1911		(ref. to mid-19C) Gouverneur As I Remember 12: ‘Pipe laying’ was an organized scheme for controlling votes, and derived its name from certain political manipulations connected with the introduction to Croton water in New York City [DA].