No matter what kind of democratic or vote-based governmental system you happen to be a part of, if you have the right to and access to voting and choose to vote, there’s something that leads to you voting for who you do. For many people, what leads to their choice to vote is a very complex issue, built around everything from gender and socialization to belief and the organization of candidates on the ballot.

In fact, the type of election you’re voting in also has an impact. According to a study in the International Proceedings of Political Science in 2006 (and as cited in Wikipedia), quote:

In national elections it is usually the norm that people vote based on their political beliefs. Local and regional elections differ, as people tend to elect those who seem more capable to contribute to their area. A referendum follows another logic as people are specifically asked to vote for or against a clearly defined policy.

For single-issue voters, however, the choice of who to vote for in even national elections can be boiled down to a single question. There’s a lot of different numbers out there about how many people fall into this category of single-issue, and I think that’s likely because it varies according to the election itself. For example, quoting from the BBC:

At a UK level, many voters in 2015, switched support to UKIP which opposed the UK’s membership of the European Union (EU). To these voters the UK’s relationship with the EU is their main concern.

In the United States, abortion may have been chosen by a coalition of conservative leaders in the 1980s as the issue around which they wanted to rally their potential supporters. Quoting from an article by reporter Katherine Stewart, quote:

The politics of abortion changed in response to two political developments in the postwar era. The first was Nixon’s “Southern Strategy”[…]. The second was the emergence of a cadre of religious conservative leaders who, alarmed by the rapid social changes of the 1960s and 1970s, shared a strong desire to form a political coalition.

Notably, the issue that first brought together the leadership of these two forces together wasn’t abortion, but rather the desire to protect racially segregated religious schools from losing their tax-exempt status. […] As conservative activist Paul Weyrich, who coined the term “Moral Majority,” has attested, at the first gatherings of the Religious Right, abortion took a back seat to school prayer, the Equal Rights Amendment, and above all, the tax status of racially segregated religious academies.

The breakthrough came when savvy religious activists, such as Weyrich and Jerry Falwell, noticed that the language used to express concerns about abortion was in essence the same language used to articulate anxieties about evolving family, gender, and racial norms. […] Once the strategy of using abortion as the unifying issue for the right was settled, theology changed dramatically in order to service this new purpose.