I've only heard of people "tactically voting" -- whereby they don't vote for their first choice, but their nearest choice that contends with an incumbent or person/party they don't want -- over the last 5-10 years. It seems like a fairly obvious tactic, so I doubt this is the case, but is this a relatively modern phenomenon?
(My question applies broadly, across the Western world and, indeed, the whole world. Especially if there's a marked difference in any particular area.)
I heard about tactical voting early in the 1990s, but as a concept, it is at least as old as Duverger's law, which dates back to the 1950s. Duverger's law says that first-past-the-post voting devolves toward a choice between two candidates. Tactical voting is the mechanism by which it does so.
You might also consider whether you mean something more specific than tactical voting. There are at least three types of tactical voting:
Burying and bullet voting are used in multiple-choice methods, e.g. IRV, etc. Compromising and push-over are used in plurality voting.
Compromising is when people really prefer one alternative but do not think that that person can win. So they pick one of the two choices that they think might possibly win. This is what Duverger's law discusses.
Push-over is when people vote for a candidate that they do not want to win in the first round so as to give the candidate they do want to win an easier opponent in the second round. For example, someone might vote for Donald Trump as the weakest of the Republican candidates in the hope that Hillary Clinton would beat him in the general election. Of course, push-over may not work, as people may underestimate a candidate's appeal or overestimate their own candidate's appeal.
Wikipedia describes an early form of tactical voting:
[Hill's] pupils were asked to elect a committee by standing beside the boy they liked best. This first produced a number of unequal groups, but soon the boys in the largest groups came to the conclusion that not all of them were actually necessary for the election of their favourite and some moved on to help another candidate, while on the other hand the few supporters of an unpopular boy gave him up as hopeless and transferred themselves to the candidate they considered the next best. The final result was that a number of candidates equal to the number required for the committee were each surrounded by the same number of supporters, with only two or three boys left over who were dissatisfied with all those elected. This is an admirable example of the use of STV.
Obviously this wouldn't scale to a larger scale election, but compromising dates back to 1821. It's not clear to me that it was recognized as such at the time, but it was used.
You could probably find earlier examples on History.SE. Any election with multiple rounds is going to involve compromising unless someone has overwhelming support.
