GOP frontrunner Donald Trump and Texas Sen. Ted Cruz are butting heads over family, a relatively surprising notion since it used to be unusual for candidates to attack the personal lives of their opponents.

This started Tuesday when Trump criticized Cruz and his followers for putting together an advertisement that included Trump's wife Melania Trump. In the ad, the anti-Trump campaigners attacked his wife for posing naked. According to BuzzFeed, this was done specifically to target family friendly voters in the recent Idaho, Utah and Arizona primary elections.

The ad, showing a naked Melania Trump over a fur rug, reads: "Meet Melania Trump. Your next first lady. Or, you could support Ted Cruz on Tuesday."

As you might expect, the ad didn't sit well with Trump. He expressed his concern in a tweet that was published, deleted and rewritten a few times Tuesday night before he posted it for good on social media.

Cruz responded to those comments on CNN, saying that his wife was out of Trump's league, and that he had nothing to do with the advertisement.

"So last night, Donald threatened my wife. He went directly after my wife," Cruz told CNN's Chris Cuomo Wednesday morning, according to The Blaze. "And I gotta tell you that, number one, Heidi, my wife, she's the daughter of missionaries in Africa, she's my best friend in the world, and if Donald wants to get in the character fight, he's better off sticking with me cause Heidi is way out of his league."

Cruz also said this incident directly shows who Trump is as a candidate, since he has blatantly attacked another candidate's wife.

"But it is also, it reveals a lot about character, reveals a lot about class, that Donald's instinct is to try to attack my wife and sully her, and you know what, that should be beneath Donald," Cruz continued. "But you know Chris, the reason he's doing that is cause he had a very bad night last night. He got clobbered in Utah."

Trump replied:

This whole issue may be more than just political warfare, though. It's often been considered a cardinal sin for presidential candidates to bring their opponents' families into the fold and attack them. As NPR reported, their private lives - especially surrounding family members - have long been considered off limits.

But that has begun to change in recent years.

"There is no definition of 'fair game,'" Republican political consultant Mary Matalin told NPR. "So whatever you think it is, you can disabuse yourself of any of that notion."

Matalin said she's followed politicians across the country and has seen their wives constantly attacked and bombarded by the media. Sometimes candidates will even attack their opponents' children, causing an even harsher rift among the family.

"It's unfair, it's irrational, it's pain that's relentless," she told NPR. "And if you can develop a defense for yourself, you can never develop a defense for your loved ones."

Still, the majority of presidential candidates throughout the years consider their families, especially their wives and children, to be off-limits. In 2008, then Sen. Barack Obama championed for less ridicule and criticism of his wife and future first lady, Michelle Obama, The New York Times reported.

Obama said at the time he wouldn't even consider going after his opponent John McCain's wife, according to The New York Times.

"I think families are off-limits," Obama said, according to The Times. "I would never consider making Cindy McCain a campaign issue. And if I saw people doing it, I would speak out against it. The fact that I haven't seen that from John McCain is, I think, a deep disappointment."

In fact, McCain's campaign at the time issued a statement agreeing with Obama, advocating for less gossip about each other's families and a more focused discussion on the political issues.

"Senator McCain agrees with Senator Obama that spouses should not be an issue in this campaign," he said.

Though McCain and Obama surely had their squabbles, this moment when the two agreed to make the 2008 presidential campaign less about their wives and more about issues is a shining moment when two politicians got along for the sake of family.

But this isn't the only moment political candidates have put their differences behind them. There have been multiple times throughout political history that politicians have acted positively towards their opponents' families, some of which we've highlighted below.

Michelle Obama and Ann Romney = #WifeGoals

In 2012, Obama sought re-election in a race against Republican nominee Mitt Romney. Before the two got together for the first 2012 presidential debate, Michelle Obama and Ann Romney shook hands and hugged. It was a touching moment, one that shows how politicians' families can get along.

Michelle Obama praises her husband's former opponent

About seven years ago, before Obama became president and around the same time he was fighting for the White House spot against Hillary Clinton, Michelle praised Clinton for creating an America where her daughters could find meaningful jobs and success in the world.

Laura Bush praises the current president's wife

In August 2014, Laura Bush spoke with the current first lady at the African Summit in Washington, saying the pair were two women in the "sorority" made up of former first ladies.

Barbara Bush praises the Clinton-Bush relationship

In an interview with CNN, former first lady Barbara Bush said she felt that former President Bill Clinton always thought of her husband - George H. W. Bush - as the father Bill Clinton never had.

Honoring a lost wife and mother

After former first lady Nancy Reagan passed away, politicians from both sides of the aisle - some opponents of the Reagan way of politics, others supporters - came together to honor the fallen former first lady, who died recently of heart failure.