Donald Trump and Ted Cruz engage in late night Twitter war as campaign descends to 'gutter politics'

The campaign has become more toxic as the race has become closer

The battle between Mr Cruz and Mr Trump has intensified as the campaign has continued (AP)

One would be forgiven for thinking that given the intensity of the Republican race, Donald Trump and Ted Cruz would made sensible use of their evenings by getting some rest.

But as a new poll showed the contest getting even tighter, the two choose to engage in a late night Twitter row, as the mud-slinging and insults – which has now dragged in their wives – increased.

The row has it origins in a campaign advert produced by an anti-Trump group not linked to Mr Cruz, which featured an image of the tycoon’s wife from a 2000 model shoot for GQ magazine. Mr Trump had accused Mr Cruz of being involved in the advert and warned that he would “spill the beans” about the senator’s wife, Heidi.

On Wednesday evening, Mr Trump retweeted an unflattering image of Mrs Cruz alongside an image of his wife, Melania.

Mr Cruz responded by tweeting: “Donald, real men don't attack women. Your wife is lovely, and Heidi is the love of my life.”

Earlier in the day, the Texas senator had told CNN: “If Donald wants to get in a character fight he’s better off sticking with me, because Heidi is way out of his league.”

Mr Trump accused Mr Cruz of stealing the line from Michael Douglas as the star of the 1995 movie The President.

“Lyin’ Ted Cruz steals foreign policy from me, and lines from Michael Douglas - just another dishonest politician,” Mr Trump tweeted.

In the film, the character played by Douglas says - referring to his girlfriend: “If you want a character debate Bob, you better stick with me because Sydney Ellen Waid is way out of your league.”

The advertisement featuring the glamour shot of Mrs Trump was produced by an anti-Trump super PAC, Make America Awesome, which has no known connection to the Cruz campaign, but which urged voters in Utah to vote for the Texas senator.

Mr Cruz won the Republican contest in Utah Tuesday, capturing more than 50 per cent of the vote and all 40 of the state’s delegates. Mr Trump won the night's Arizona primary.

“When Donald gets scared, when he gets angry, when he gets threatened ... So last night Donald threatened my wife, he went directly after my wife,” Mr Cruz said. He said Mr Trump was engaging in "gutter politics".

Mrs Cruz is a senior executive for Goldman Sachs, and took a leave from work in order to help her husband's campaign. She has become one of the campaign's most accomplished fundraisers.

Mrs Cruz also defended herself against Mr Trump, saying: “We are not worried in the least. I have one job in this campaign and that is to help Ted win this race. I think it is the easiest job in the world.

“All I have to do is speak the truth about what I know about my husband and our family.”

The spat came as a new poll from Fox News found that Mr Cruz and John Kasich are ahead of Hillary Clinton in one-on-one match-ups, but Mr Trump trails the Democratic front runner by 11 points.

The poll said Mr Kasich was the Republican's best shot of beating Ms Clinton, In a national poll of the Republicans, Mr Trump remains in the top spot with 41 per cent support, while Mr Cruz has 38 per cent. Mr Kasich gets 17 per cent.