Ted Cruz
Ted Cruz: He's really 'ticked off' now. Getty Images

Just when you thought the GOP presidential race couldn't get any low down and dirtier, Ted Cruz has called Donald Trump a "snivelling coward," and warned him to "stay away" from his wife, Heidi. It's been a week of the candidates battling over their wives, beginning with a threat from Trump to "spill" some mysterious "beans" about Heidi Cruz after Trump thought Cruz used a nudie of Melania Trump from a GQ shoot in a political ad.

Cruz has denied he had anything to do with the ad. Then The Donald posted an unflattering photo of Heidi next to a glamourous shot of Melania, saying: "Forget spilling the beans, a picture is worth a thousand words."

"I have to say, seeing him go deeper and deeper into the gutter, it's not easy to tick me off," Cruz said at a news conference in Wisconsin, reported NPR. "But you mess with my wife, you mess with my kids, it'll do it every time.

"Donald, you're a snivelling coward," Cruz said. "Leave Heidi the hell alone." Oh, and he also criticised Trump's "conservative credentials." Cruz wasn't the only one ticked off by the candidates' back-and-forth.

"Talk about things that people really care about, and knock this crap off because these are serious times, and you're not behaving like you want to be president of the United States," said Republican South Carolina Senator and one-time presidential candidate Lindsey Graham.

He has rather gone off the Grand Old Party. My party is "absolutely screwed up," Graham conceded on The Daily Show. When Graham was recently asked if he preferred Trump or Cruz, he responded: "It's like being shot or poisoned."

If it "were up to me, we wouldn't nominate anyone. We'd start all over again," he said. Graham — finally and reluctantly — endorsed Ted Cruz, and said that he's now "on the Ted train."

"He's not completely crazy," Graham said of Cruz. "Partially crazy?" asked Daily Show host Trevor Noah. Graham fired back: "That works in Washington."

In February Graham said: "If you killed Ted Cruz on the floor of the Senate, and the trial was in the Senate, no one would convict you." That, he said on the Daily Show, tells you "all you need to know about Donald Trump."