Heidi Cruz, the wife of Republican presidential candidate Ted Cruz, denied March 23 that her husband's campaign put out an ad featuring a nude image of Donald Trump's wife, Melania. (Reuters)

As Republicans in Utah waited for their caucuses to start on Tuesday night, Republican front-runner Donald Trump — who had lagged behind in the state's polls and went on to lose Utah — tweeted an attack on his chief rival that got personal.

“Wow Sen. Ted Cruz, that is some low-level ad you did using a picture [of] Melania in a G.Q. shoot. Be careful or I will spill the beans on your wife," Trump tweeted 17 minutes before the Utah caucuses were scheduled to start.

Within seconds, Trump deleted the tweet. Ten minutes later, he posted a more polished version of his seemingly impromptu attack: "Lyin' Ted Cruz just used a picture of Melania from a G.Q. shoot in his ad. Be careful, Lyin' Ted, or I will spill the beans on your wife!"

"Be careful" is what Trump often utters or tweets when he feels threatened by a rival, a super PAC or a new line of attack. It's his way of reminding those involved that he doesn't abide by the traditional rules of political decorum, and he will not only go after opponents in shockingly belittling ways — he will do the same to their spouses, parents, relatives and loved ones.

"BE CAREFUL," Trump warned Democratic front-runner Hillary Clinton in late December when she accused him of having "a penchant for sexism" after he accused her of lacking the strength and stamina required for the presidency. Trump then accused former president Bill Clinton of having a "terrible record of women abuse," prompting several days of rehashing Clinton's history of womanizing and allegations of abuse. On the campaign trail since then, Trump has bragged that he gave the political power couple a very bad weekend, and has warned that his attacks in a general election would pale in comparison.

Heidi Cruz has worked at the White House and Goldman Sachs, but she's hoping to add one more item to her resume: U.S. first lady. (Sarah Parnass/The Washington Post)

Ahead of the Iowa caucuses, former first lady Barbara Bush filmed a video message to the supporters of her son, former Florida governor Jeb Bush, that seemed to take a veiled shot at Trump in this line: "When push comes to shove, people are going to realize Jeb has real solutions, rather than talking about how popular they are, how great they are." Trump then tweeted in late January that Bush "desperately needed mommy to help him" — an attack that he would repeat over and over again until Bush dropped out of the race. Bush fired back with a photo of his mother in football pads with this note: "I'd be careful Donald."

But Trump continued to use Bush's "mommy" to belittle his rival, while saying that he was sure she was a lovely woman. Ahead of the South Carolina primary, Trump also suggested at a GOP debate that Bush's brother, former president George W. Bush, should have been impeached. That same week, Trump mocked Jeb Bush's low poll standings and taunted: "Go home. Go home to Mom. Go home to Mommy."

On Tuesday night, Trump threatened to tell all about Cruz's wife because of an ad circulating on social media that showed a 15-year-old photo of Melania Trump posing seemingly nude on a blanket of fur for a magazine photo shoot with this message: "Meet Melania Trump. Your next first lady. Or, you could support Ted Cruz on Tuesday." The ad is the work of an anti-Trump super PAC that calls itself "Make America Awesome" and this week targeted Mormon voters in Utah with a series of social media ads.

Cruz quickly pointed out on Twitter Tuesday night that he is not tied to the ad: "Pic of your wife not from us. Donald, if you try to attack Heidi, you're more of a coward than I thought. #classless." Trump responded on Twitter on Wednesday morning: "Lyin' Ted Cruz denied that he had anything to do with the G.Q. model photo post of Melania. That's why we call him Lyin' Ted!" Later in the morning, top Trump adviser Stephen Miller said in an interview on CNN that Cruz should "tell his super PACs to stand down."

Trump defends these sorts of attacks by saying that he's simply defending himself and implying that it's the fault of his rivals for putting their loved ones in a position to be attacked: Clinton accused him of sexism, so he went after her husband for being sexist. Barbara Bush made a veiled attack at him in a video, so he used her to mock her son. Melania Trump's modeling photo was used to encourage Utah voters to pick Cruz, so he threatened to share unflattering information about Cruz's wife.

Often these attacks are vague and open-ended, but they signal to Trump's supporters that they should feel free to launch their own attacks. After Trump threatened to "spill the beans" on Heidi Cruz on Tuesday night, Twitter filled with speculation about what those beans might be. There were links to a heavily redacted 2005 Austin Police Department report detailing an officer finding Heidi Cruz next to an expressway and deciding she was a "danger to herself," along with links to articles about her Wall Street career and a slew of unflattering hashtags.

The power of this social media barrage is not to be underestimated and is well recognized by the Trump campaign. As Trump tried to get Megyn Kelly booted from a debate in late January, which he eventually skipped, Fox News put out a statement saying that Trump's campaign manager said in a phone call with a Fox News executive "that Megyn had a ‘rough couple of days after that last debate’ and he ‘would hate to have her go through that again.' "

Late Tuesday night, former Trump campaign adviser Roger Stone tweeted: "Melania HOT, Heidi NOT."

Stone, who remains a confidant to Trump, included a link in the tweet to a column he wrote for the Daily Caller last week that accused the Cruzes of having "a sometimes troubled relationship" and accused the presidential candidate's parents of having a "troubled marriage ... plagued by alcohol and infidelity." The column detailed the incident in Austin and noted that Heidi Cruz is receiving help from her mother-in-law and a live-in nanny in caring for her children.

Cruz was on the "Today" show on Wednesday morning and defended his wife against the attack Trump had technically not yet launched, calling him "a bully." Heidi Cruz then addressed the situation herself during a campaign stop in Wisconsin, saying that the photo of Melania Trump that sparked the Twitter war the night before was not circulated by her husband's campaign.

“In no way, shape, or form were we related to it," Heidi Cruz told a small group of reporters at the campaign's office in Waukesha. "And we are really excited today about the incredible results from yesterday.”

As Cruz tried to nudge the conversation to the campaign’s strategy and win record, reporters kept asking about the tweet.

How did she feel when she saw it? “You know, as I said, you probably know by now that most of the things Donald Trump says have no basis in reality,” she said.

What message did she think the tweet sent to the women of Wisconsin? “What I think most women want is a better future for their children,” she responded.

Did she think that candidates' spouses should be off limits? “Like I said, we have run our campaign on the principles that Ted and I believe in,” Cruz said. “And a lot of the things that are done from time to time are not from our campaign. So I want you to focus on what our campaign puts out, a positive, hopeful, optimistic agenda for this country.”

David Weigel and Philip Rucker contributed to this report.