Donald Trump questions whether Cruz can be president
Republican presidential front-runner Donald Trump has questioned whether rival candidate Ted Cruz is eligible to become his party's nominee because he was born in Canada.
Mr Trump called it a "very precarious" issue for the party and said that Mr Cruz's nomination could be challenged in court.
Mr Cruz was born in Calgary to an American mother and a Cuban father.
Most legal experts believe Mr Cruz meets the requirements to be president.
The Cruz campaign responded by saying Mr Trump had "jumped the shark" - a pop culture reference to when a TV show or fad has overreached and fallen into decline.
The Texas senator is performing well in polling in the early voting state of Iowa. While Mr Trump leads most polls, several surveys show Mr Cruz is the top choice of likely Republican voters.
Candidates for US president must:
- be a "natural born citizen" - interpreted as being born in the US or having one parent who is a US citizen
- be 35 years of age or older
- live in the US for the past 14 years
Much of Mr Trump's support comes from independents, disillusioned Democrats and people who have never voted before.
Some analysts are predicting that Mr Cruz will win because of how the voting process is structured in Iowa.
The caucus format requires more time and dedication from voters than a typical US primary election.
Before Mr Trump became a presidential candidate, the New York businessman had repeatedly questioned President Barack Obama's citizenship.
Mr Obama was born in Hawaii to an American mother and a Kenyan father.
Authorities in Hawaii and Mr Obama both provided detailed birth records after some conservatives such as Mr Trump questioned where he was born.
Mr Obama's allies have said the "birther" movement was a racist effort to discredit the county's first black president.