WASHINGTON • Mr Donald Trump and his main challenger for the Republican presidential nomination, Senator Ted Cruz, have escalated their bitter feud, taking to social media to clash over two unlikely figures: their wives.
An anti-Trump political group unveiled a controversial campaign ad ahead of Tuesday's votes in Arizona and Utah that uses a photograph of Mr Trump's wife, Melania, lying provocatively in his custom-fitted jet, naked and handcuffed to a briefcase.
"Meet Melania Trump, your next First Lady," read the online ad, posted on Facebook by the group Make America Awesome.
"Or, you could support Ted Cruz on Tuesday."
Mrs Melania Trump, 45, is a Slovenian-American jewellery designer and former model.
The use of the image, from a GQ magazine photo shoot in 2000, when she was Melania Knauss and not yet married to Mr Trump, angered the billionaire real estate magnate, who made his own veiled threat about Mr Cruz's wife, Heidi, in a tweet late on Tuesday.
"Lyin' Ted Cruz just used a picture of Melania from a GQ shoot in his ad. Be careful, Lyin' Ted, or I will spill the beans on your wife!" Mr Trump posted to his seven million Twitter followers.
Mr Cruz wasted no time firing back. "Pic of your wife not from us," he tweeted. "Donald, if you try to attack Heidi, you're more of a coward than I thought. #classless."
The suggestive ad was apparently part of an effort to sway voters in predominantly Mormon Utah by appealing to the morality of the state's socially conservative population, according to The New York Times.
Mr Cruz comfortably won Utah, and Mr Trump finished third, behind Ohio Governor John Kasich. Mr Trump easily won Arizona in the day's other Republican contest.
Mrs Heidi Cruz, 43, is an investment manager who has taken a leave of absence from her position at Goldman Sachs to hit the campaign trail with her husband.
She swatted away Mr Trump's threat, saying she was "not worried" about his bluster.
"You probably know that most of the things that Donald Trump says has no basis in reality," she told reporters in Wisconsin, which votes on April 5.
"We have run our campaign with the principles that Ted and I believe in, and a lot of the things that are done from time to time are not from our campaign."
Meanwhile, Mr Cruz scored the critical endorsement of former Florida governor Jeb Bush on Wednesday, after the Republican and Democratic front runners scored key victories in the western state of Arizona.
After dropping out of the presidential race last month following a disappointing campaign, Mr Bush, whose father and brother are both former presidents, said the Republican Party has to unite or face certain defeat in the election, probably to Mrs Hillary Clinton. In doing so, he took a pot shot at front runner Mr Trump, who leads Mr Cruz comfortably in the polls.
"For the sake of our party and country, we must overcome the divisiveness and vulgarity Donald Trump has brought into the political arena or we will certainly lose our chance to defeat the Democratic nominee, most likely Hillary Clinton, this fall," the statement said.
"That is the only way we can reverse President Barack Obama's failed domestic and foreign policy agenda and turn our country around."