Donald Trump questions rival Ted Cruz's eligibility to run for the US presidency

In some polls, the senator from Texas has pulled ahead of Mr Trump in Iowa

Ted Cruz and his daughters (AP)

Striving to retain his lead in the scramble for the Republican nod for president, Donald Trump has delved into his old bag of “birther” tricks to tackle Senator Ted Cruz, saying he may create “very precarious” legal obstacles for the party as its nominee because he was born outside the United States.

The new attack effectively shatters any lingering notion that Messrs Trump and Cruz can keep it cordial as they find themselves in a near death-match to corner the party’s crucial conservative wing ahead of the Iowa caucuses which are now just four weeks away. Some polls show Mr Cruz leading in the state.

Mr Trump made his first political splash a few years ago with his long-running birther campaign, beloved by the right, querying the nationality of President Barack Obama. It ended in 2011 after Mr Obama released the long-form version of his birth certificate confirming Hawaii as his birthplace.

Donald Trump might even be getting stronger (Reuters)

It is true that Mr Cruz was born in the Canadian city of Calgary in 1970 and his father was Cuban. But his mother is an American, which to most scholars is enough to make him a “natural born” US citizen qualified to run for president. However the issue has never been settled by the Supreme Court.

With the clock ticking to Iowa and the New Hampshire primary that comes just days later, the tone on the trail continues to sharpen. Targeted in new TV spots from Marco Rubio, the Governor of New Jersey, Chris Christie, accused the Florida Senator of trying to “slime his way to the White House”.

Mr Rubio, also the son of a Cuban immigrant, found himself mocked by Mr Cruz’s campaign because of his choice of footwear – generally cowboy boots with generous heels. “A Vote for Marco Rubio Is a Vote for Men’s High-Heeled Booties,” Rick Tyler, Mr Cruz’s communications director, said in a Tweet.

Senator Ted Cruz, Republican presidential candidate and Tea Party favourite, is welcomed at a campaign rally in Las Vegas this week (AP)

Widely seen as Mr Trump’s most potent rival, Mr Cruz made light of the tycoon’s birther remarks. His campaign responded with a clip from Happy Days, the vintage TV show, featuring its lead character, Fonzie, jumping over a shark on water skis. The stunt was seen as a bid to hold up ratings and “jumping the shark” quickly became synonymous with last-gasp desperation.

“One of the things that the media loves to do is gaze at their navels for hours on end by a tweet from Donald Trump or from me or from anybody else. Who cares?” Mr Cruz said at a rally, going on: “The best way to respond to this kind of attack is to laugh it off and move on to the issues that matter.”

While conservatives loved the birther issue on Mr Obama, it isn’t clear that using the same tactic on Mr Cruz will have anything like the same effect. The Texas senator has a strong hold on the right of the party, in part because of his record of trying to repeal Obamacare up to and including triggering a brief shutdown of the federal US government in 2013.

In television interviews yesterday Mr Trump sought to keep the focus on the birther issue. “It’s a problem for him, and it’s a problem obviously for the Republicans,” Mr. Trump said on MSNBC. “Let’s assume he got a nomination and the Democrats bring suit, the suit takes two to three years to solve, so how do you run?”

He first broached the question with the Washington Post, saying Mr Cruz’s birthplace risked becoming a genuine problem moving forward. “It’d be a very precarious one for Republicans because he’d be running and the courts may take a long time to make a decision. You don’t want to be running and have that kind of thing over your head.”