One common use of 'plumping' in the nineteenth century was when an elector used only one vote at a general election. Most constiuencies had two MPs so if (for example) the Liberals put up two candidates but the Tories only had one, a Tory loyalist would simply plump for his party's candidate and not use his second vote as this would mean voting for a Liberal. Poll books often listed the 'plumpers' as a distinct category.