Republican presidential hopeful Ted Cruz was endorsed by former rivals Jeb Bush and Scott Walker (AP)

Republican presidential candidate Ted Cruz has received a boost from former contenders Scott Walker and Jeb Bush, who say the Texas senator is the party's best - and last - hope for stopping front-runner Donald Trump.

Ahead of Wisconsin's April 5 primary contest state governor Mr Walker, who dropped out of the race last year, declared that only Mr Cruz could catch Mr Trump as time runs short in the primary season.

And former Florida governor Mr Bush gave Mr Cruz his endorsement - a step perhaps designed to hurt Mr Trump more than help the billionaire businessman's rival.

"For the sake of our party and country, we must move to overcome the divisiveness and vulgarity Donald Trump has brought into the political arena," said Mr Bush, who was knocked out of the 2016 contest last month.

"To win, Republicans need to make this election about proposing solutions to the many challenges we face, and I believe that we should vote for Ted as he will do just that."

As Democrat front-runner Hillary Clinton addressed rising national security concerns, the Republican contest was hit again by personal insults, this time involving the candidates' families.

Mr Cruz slammed Mr Trump during an appearance in Mr Trump's home town for making a vague threat on Twitter the night before to "spill the beans" on Mr Cruz's wife. "Gutter politics," Mr Cruz replied.

Mr Trump's warning that he would disclose something about Heidi Cruz came in response to an advert by an outside political group that featured a provocative photo of Mr Trump's wife Melania when she was a model and before they were married. Mr Trump mistakenly identified the Cruz campaign as the source of the ad.

Heidi Cruz addressed the situation directly during an appearance outside Milwaukee, saying: "The things that Donald Trump says are not based in reality."

The Republican in-fighting came the day after Mr Cruz scored a win in Utah and Mr Trump claimed Arizona. Despite modest signs of strength, the first-term Texas senator needs a near miracle to catch the billionaire businessman.

The day-after delegate maths laid bare the challenge: Mr Cruz needs to win 83% of the remaining delegates to overtake the front-runner. And further complicating his path, Ohio governor John Kasich vowed to stay in the race at least until the next primary.

Things were decidedly less contentious on the Democratic side.

Mrs Clinton won in Arizona, maintaining a lopsided advantage over Bernie Sanders in the Democratic race despite his wins in Utah and Idaho on the same night. The former US secretary of state is now almost three-quarters of the way to the Democratic nomination.

As the world grapples with a new wave of overseas violence, Mrs Clinton engaged Mr Trump on national security with a California speech painting him as a misfit as potential commander in chief and laying out what she would do to keep America safe in perilous times.

"Slogans aren't a strategy. Loose cannons tend to misfire. What America needs is strong, smart, steady leadership," she said.

Mr Trump's win in Arizona gave him a little less than half the Republican delegates allocated so far. He needs to win 54% of those remaining to claim his party's nomination before the July convention.

PA Media