Bill Clinton's sexual history is fair game for Donald Trump. But it's bad politics.

All's fair in love and war, but how about politics?

At the moment, 2016 watchers are wondering whether certain things are "fair game" or not, which is another way of asking whether we should object not just to the substance of an attack, but to the attack's subject matter itself. Thanks to Donald Trump, the immediate topic is Bill Clinton's sexual history, but this probably won't be the last time the "fair game" question arises.

In case you aren't following Trump's campaign closely, this has come up because Hillary Clinton criticized him for some of his more sexist comments, whereupon he hit back by saying that Bill Clinton had affairs. "You look at whether it's Monica Lewinsky or Paula Jones or many of them," Trump said. "That certainly will be fair game. Certainly if they play the woman's card with respect to me, that will be fair game." So one reporter and pundit after another is asking, "Is this fair game?"

When we say something is "fair game," we aren't just talking about fairness, which has to do with things like adherence to rules, honesty, and equitable treatment. We're really asking whether a particular topic should go completely unmentioned in the course of the campaign.

For instance, politicians' children are usually considered not fair game, because they don't choose to be part of the political process. Last week The Washington Post (one of my other employers) pulled a cartoon that depicted Ted Cruz's children as trained monkeys, a commentary on a Cruz ad in which his children participated. The cartoon was universally criticized, because everyone seems to agree that while a candidate using his children for political purposes is fair, mocking the candidate for it in a way that also appears to mock the children is not.

Other topics are considered fair game by some but not by others, like a politician's religious beliefs (I've always held that they should matter, and be debated, in proportion to however much the candidate him/herself offers their religiosity up as a reason to vote for them). Similarly, the more a spouse is an active participant in the campaign, the more he or she is fair game for criticism. And nobody is fairer game than Bill Clinton, who is, after all, a former president.

And since Hillary Clinton often mentions her husband's presidency as an example of the kind of successful approach she would bring, that presidency — warts and all — is certainly relevant. But if Republicans want to re-litigate the Monica Lewinsky matter, they probably shouldn't hope that things will turn out differently this time. You may recall that they were unable to remove Clinton from the presidency, and two years after being impeached he left office with approval ratings in the high 60s. In the end, the public decided that though his private behavior was deplorable, they were happy with the job he was doing as president. They also concluded that a bunch of prurient Republicans had become positively obsessed with Clinton's sexual life and dragged the country through a needless impeachment crisis.

It's fair game to talk about all that again (which, I must point out, members of the media would absolutely love to do). What's much harder to figure out is why Bill Clinton's behavior provides a reason to vote against his wife. That's the substance of the question, which still awaits an explanation.

One might even ask what relevance Donald Trump's obvious sexism has for the presidency. Unlike with some of the other large groups he has alienated, it's less clear what the connection would be between Trump's sexism and his actual policy positions. Yes, he finds women's bodily functions "disgusting," in the word he repeatedly uses (see here or here), and has a history of dumping his wives when they hit their 40s so he can get himself a younger model. But his positions on issues of particular concern to women are little different from those of most Republicans, even those who are perfectly polite and respectful to everyone (you can argue that things like opposing abortion rights are inherently sexist, but that doesn't tell us anything about Trump specifically).

As I've noted before, contemporary campaigns as they play out in the media are mostly about what the candidates (and occasionally those who support or work for them) are saying. We go through a perpetual cycle of utterance and umbrage, where one candidate says something, their opponents claim to be outraged by it, then reporters and pundits spend a few days mulling over how bad it really was, and then it starts all over again with the next shocking statement.

That isn't to say that some statements aren't genuinely shocking and revealing; talking about what candidates are talking about can provide useful insights. But unless there are particular innocent people (like children) being harmed, almost any topic ought to be fair game in a campaign. If I'm running for office and I want to allege that my opponent's great-great-grandfather was a low-down dirty horse thief, my attack should persuade no one. But that not because great-great-grandad Ukariah ought to be off-limits, it's because it's idiotic to care one way or the other.

There are also plenty of attacks that are problematic while the topic they address is still fair game. For instance, when Republicans alleged that Barack Obama was "palling around with terrorists" (in Sarah Palin's evocative phrase) because he had a couple of acquaintances who were former 1960s radicals, the problem wasn't that the people Obama associated with wasn't a legitimate topic of inquiry. The problem was that Republicans spun incidental contacts into absurd conspiracy theories and vicious attacks on Obama's patriotism. An attack can be unfair — dishonest, distracting, irrelevant — while the subject it concerns can still being fair game at a more fundamental level.

So if Donald Trump wants to say that people shouldn't vote for Hillary Clinton because of what Bill Clinton did with Monica Lewinsky, he should go right ahead. It's fair game. He just shouldn't expect it to convince anybody.