Politicians' complaints about media coverage are usually self-serving and often bogus. How many times have we heard Donald Trump charge that news outlets never show his huge crowds? It's a myth.

But Marco Rubio aired a media grievance during Saturday's Republican presidential debate that has real merit. Conservative commentator Mary Katharine Ham, a panelist for the event, asked the senator from Florida how he plans to speak to millennial voters about his opposition to same-sex marriage and abortion while defending against inevitable accusations of intolerance. Rubio didn't actually address his millennial strategy on these contentious social issues, but he did say this:

Here's what I find outrageous: There has been five Democratic debates. The media has not asked them a single question on abortion; and on abortion, the Democrats are extremists. Why doesn't the media ask Hillary Clinton why she believes that all abortion should be legal, even on the due date of that unborn child? Why don't they ask Hillary Clinton why she believes that partial-birth abortion — which is a gruesome procedure that has been outlawed in this country — she thinks that's a fundamental right. They are the extremists when it comes to the issue of abortion, and I can't wait to expose them in a general election.

I'm not sure it's accurate to say Clinton believes all abortion should be legal. But really, that's part of the problem Rubio highlighted. Even after Clinton's many decades in the public eye, her position on abortion remains a bit hazy.

Here's what Clinton said when partial-birth abortions came up in a debate during her Senate run in New York in 2000:

I have said many times that I can support a ban on late-term abortions, including partial-birth abortions, so long as the health and life of the mother is protected. I've met women who faced this heart-wrenching decision toward the end of a pregnancy. Of course it's a horrible procedure. No one would argue with that. But if your life is at stake, if your health is at stake, if the potential for having any more children is at stake, this must be a woman's choice.

That response doesn't quite line up with Rubio's characterization; Clinton indicated she supports late-term partial-birth abortion as an option — but only in situations where a mother's health is in jeopardy. She said she supports a ban in other cases.

This was a long time ago, of course, and abortion is an issue on which politicians have a tendency to "evolve." Where does Clinton stand today? It's a little unclear.

Last year, early in the presidential race, Sen. Rand Paul (R-Ky.) called on Democrats to answer whether it is, as he put it, "okay to kill a seven-pound baby in the uterus." Paul's challenge was part of a clumsy response to questions about his own abortion stance — which he declined to spell out in detail. But setting hypocrisy aside for a moment, it's a reasonable thing to wonder. His inquiry drew a sharp retort from Democratic National Committee Chairman Debbie Wasserman Schultz (Fla.) but not a comprehensive answer.

And Clinton didn't offer any clarity, either. The conservative Weekly Standard magazine followed up Paul's remarks by inquiring about Clinton's position on late-term abortion. It reported that her campaign ignored all questions.

It's not so easy to blow off a tough question in a debate, which is why the topics that moderators decide to raise are so important. But as Rubio rightly pointed out, abortion hasn't been a topic in any of the five Democratic showdowns so far. That's despite Clinton's renewal early last month of her support for repealing the Hyde Amendment, which prohibits federal funding of abortions.

Meanwhile, Ham's question to Rubio made Saturday's Republican debate the third out of seven in which moderators have raised the subject of abortion. The most extensive discussion came in the very first debate, back in August, when Fox News Channel's Megyn Kelly asked Wisconsin Gov. Scott Walker, who opposes abortion in all instances, whether he was "too out of the mainstream on this issue to win the general election" -- similar to the line of questioning posed to Rubio on Saturday. Former Arkansas governor Mike Huckabee, Rubio and Trump were also pressed on their abortion views in that debate.

Abortion certainly hasn't dominated any GOP debate, but on several occasions Republicans have had to explain their positions. Democrats have not.

Speaking in Manchester, N.H. on Feb. 5, Democratic presidential candidate Bernie Sanders vowed to defend funding for Planned Parenthood if he's elected to the White House in November. (Reuters)

Clinton's argument against the Hyde Amendment is that it discriminates against poor people, some of whom cannot afford to exercise the abortion rights protected under Roe v. Wade without government assistance.

Even the liberal news site Think Progress noted that it is "very unusual for Democratic politicians to focus on the issue on the campaign trail."

Advocating for taxpayer funding for abortion has long been controversial, even among pro-choice lawmakers. Maintaining this division between federal dollars and abortion services was a political concession that Democrats made decades ago. Since then, the Hyde Amendment has spawned similar restrictions banning abortion coverage for government employees, Peace Corps volunteers, federal inmates, military personnel and Native American women; it's simply become the status quo — and, in many cases, is seen as a reasonable compromise.

A Marist poll published last month that surveyed a wide range of abortion-related issues found 68 percent of Americans — including a slight majority of those who self-identify as "pro-choice" — oppose the use of tax money to pay for abortions.

The "extremist" label used by Rubio is hard to define and seems a little hyperbolic here, but on the Hyde Amendment, anyway, Clinton is demonstrably out of sync with an overwhelming majority of the electorate. Given a president's power to nominate Supreme Court justices who can shape abortion laws, it's important to know where she and all candidates come down on other abortion questions, too.

On this much, Rubio is right: The media, in general — and debate moderators, in particular — should demand more answers. Just as we focus on where those who oppose abortion rights would grant exceptions, we should focus on where those who support them draw the line.