Don’t we want to test turnout, to ensure that a large part of the electorate participates in a referendum?

The problem with a simple turnout threshold is that it may actually give some voters an incentive not to vote. Tactical abstention.

Is that such a big problem?

If one side is set to lose, it can try to sabotage the referendum by abstaining so that turnout stays below the threshold. The other side is robbed of its victory.

Isn’t such a boycott a sign that the referendum is too controversial to pass?

Maybe the referendum should have been held differently or not at all. That is a different issue. Given that there is a referendum, the result should be established by letting everyone vote honestly according to their preference.

Still, the losing side is under no obligation to help the winning side. If the winning side cannot muster enough support to pass the threshold, the referendum ought not to pass.

But turn that scenario around for a second. Isn’t it strange that the referendum would pass if the losing side did turn out to vote? It is perverse that the best course of action may not be to vote for the option you prefer. This stimulus also works against the civic responsibility of voters to participate in the democratic process.

If the losing side does participate, the other side wins because it receives more votes. I don’t see how this produces wrong outcomes.

Except that tactical abstention may backfire when the race is close. If enough voters abstain to lose the referendum but not enough to invalidate it, the outcome may be the opposite of what people really wanted.

If the race is close, how do we know whether this is what happened and the winning side doesn’t actually enjoy more support?

Exactly. The fact that it is impossible to know how many people abstained for tactical reasons undermines the legitimacy of the referendum, since the losing side can call into question its outcome by claiming the support of those who abstained. Abstention should really just mean acceptance of either outcome.

What is the alternative? Isn’t tactical voting an unavoidable element of elections?

Generally speaking, maybe, but there exists an easy fix for tactical abstention. It is like you said: the losing side shouldn’t be helping the winning side pass the turnout threshold. The threshold should not take into account all votes, but only those of the winning side.

Won’t that just always make it hard for the winning side to pass the threshold?

The treshold has to be lowered accordingly. 40% could become 25% — just as with normal thresholds, choosing a specific percentage is somewhat arbitrary.

Why does this eliminate tactical abstention?

It makes it so that if you support one side, then regardless of what other voters do, the best thing you can do is vote for that side. If your side wins, you will have contributed towards reaching the threshold. If the other side wins, you won’t have. Your vote cannot help the other side, so abstaining only hurts your own cause.