The feud between Republican presidential contenders Donald Trump and Ted Cruz over their spouses heated up on Thursday (March 24) with Cruz calling the party front-runner "a sniveling coward" for threatening his wife on Twitter.Cruz bashed Trump for making Cruz's wife, Heidi, a target of social-media barbs."Donald, you're a sniveling coward," Cruz said. "Leave Heidi the hell alone."Cruz's remarks were the latest burst of hostility between the two camps, which earlier this week erupted into full view when Trump accused Cruz of posting a nude photo of Melania Trump on Twitter. Trump responded by threatening to "spill the beans" on Cruz's wife, Heidi.Cruz denied having anything to do with the image, which was part of an attack by an anti-Trump Super PAC, Make America Awesome.