Sen. Ted Cruz Sunday called questions about his eligibility to be president a "circus sideshow," saying the whole question of what constitutes natural-born citizenship has been settled. Cruz, R-Texas, who is second to business mogul Donald Trump in the race for the Republican presidential nomination in national polls but leading in Iowa, was born in Canada.

Trump challenged Cruz’s eligibility, saying Saturday night Cruz would be a target for Democrats if he got elected. But on CNN’s “State of the Union,” Cruz said his 81-year-old mother was born in Wilmington, Delaware, making him eligible to hold the office. His father was from Cuba.

“The child of a citizen born abroad is a natural-born citizen,” Cruz said, saying the question is a legal one. He then cited other presidential candidates who were not born on U.S. soil but whose parents were U.S. citizens, including John McCain in 2008, who was born in Panama, and Barry Goldwater in 1964, who was born in Arizona before it became a U.S. state.

"If Ted is the nominee, he will be sued by the Democrats," Trump said in an interview aired Sunday on "Meet the Press." "I would want the Supreme Court to rule because it hasn't ruled."

Trump started challenging Cruz’s eligibility after the real estate mogul fell behind in polling in Iowa ahead of the Feb. 1 caucuses. He said in a tweet it’s “wrong” to say the issue is a “settled matter.” Trump was among those who questioned Barack Obama’s eligibility to be president because his father was from Kenya and has questioned whether the president actually was born in Hawaii.

Cruz said he doesn’t think Americans are interested in such debates, calling them “rabbit trails and circus sideshows.”

“Most Americans couldn’t care less about a bunch of politicians bickering like schoolchildren,” Cruz said. “What the American people are focused on are the real problems facing the country … The American people are looking for who’s ready to be commander in chief.”

Cruz said he intends to make the presidential campaign a referendum on repealing healthcare reform and reforming the tax system to impose a flat tax. He predicted the 2016 election will resemble what happened in 1980 when Ronald Reagan was elected, saying Republicans need to paint in “bold colors” to win crossover votes. He accused Democrats of waging a war on the Second Amendment and basic liberties.

On immigration, Cruz said he has no intention of sending federal agents door-to-door to find undocumented immigrants and deport them. He said deportations will occur as a result of enforcement of the e-Verify and criminal justice systems.

“I don’t intend to send jackboots to knock on every door in America. That’s not how we enforce the law for any crime,” Cruz said.