Front-runner GOP candidate Donald Trump on Sunday kept up the doubts he has raised about rival Ted Cruz’s eligibility for the White House and stepped up his depiction of the Texas senator as a flip-flopper.

Trump reiterated on “Fox News Sunday” that the Supreme Court never has defined what “natural born citizen” means in the constitutional provision outlining the requirements to serve as president. He said Cruz, who was born in Canada, should get a firm answer for his own good — and the good of his party.

“What I told Ted to do is to go into court and ask for a declaratory judgment,” he said. “Ted has to get this problem solved.”

In an appearance on NBC’s “Meet the Press,” he said: “From Ted’s standpoint and from the party’s standpoint, he has to solve this problem. Because the Democrats are going to sue him if he’s the nominee.”

During an appearance on CNN’s “State of the Union,” Cruz said the law is clear: Since his mother was an American citizen, he automatically became an American at birth and is eligible to be president.

“And the dynamic that’s happening is interesting,” he said. “You know, three weeks ago almost every Republican candidate was attacking Donald Trump, today almost every Republican candidate is attacking me, and that kind of suggests maybe something has changed in the race.”

But it no longer is just Trump raising the issue. Other Republican candidates have jumped on the bandwagon. Kentucky Sen. Rand Paul agreed with Trump on “Face the Nation” on CBS that the Democrats are sure to challenge Cruz if his wins the nomination.

“If he were the president, he would be the first president not born in the United States,” he said. “And so that alone would be extraordinary. And so people have to decide for their own own minds if makes a difference.”

Florida Sen. Marco Rubio, appearing on “This Week with George Stephanopoulos,” went after Cruz not on his eligibility to run but on the issues. He stood by his recent contention that if ISIS had  lobbyists, they would have lobbied for a bill Cruz supported ending a government-run surveillance program.

Rubio also attempted to muddy the waters on immigration, an issue where he has been vulnerable because of his participation in the Gang of Eight in the Senate in 2013 to grant eventual citizenship to millions of illegal immigrants.

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“And the truth is, when it comes to Ted, he has changed his position on immigration all over the place,” Rubio said. “I mean … he used to be for birthright citizenship; now he says he’s against it. He used to for legalizing people that were here illegally. Now he says he’s against it. He used to be for a 200 percent increases in green cards, doubling them; now he says he wants none. And he said he used to be for a 500 percent increase in guest workers. And now he says he wants zero.”

Trump said on “Meet the Press” that Rubio “has a lot of good points” in his critique of Cruz. He tied the senator to the immigration policies of former President George W. Bush, for whom Cruz was a low-level staffer. On “Fox News Sunday,” he said he would be better than Cruz on immigration, security, the military and the economy.

“Ted was actually weak on illegal immigration … Nobody can compete with me on illegal immigration,” he said. “Nobody.”

Cruz rejected suggestions that he is somehow soft on illegal immigration. He noted that he modeled his platform on the issue on the ideas of the two of the biggest immigration hawks in Congress, Rep. Steve King, R-Iowa, and Sen. Jeff Sessions, R-Ala.

“We have law enforcement authorities that enforce our laws,” he said on CNN. “Your question, you keep saying that they are going to knock on every door in America. No, I don’t intend to send jack boots to knock on your door and every door in America. That’s not how we enforce the law for any crime.”

Not all of the Republican crossfire involved Trump and Cruz. Rubio continued his attack on Christie for past support for gun control, the Common Core education reform and other issues. “I mean, the last thing we need in our Republican nominee is someone who agrees with Obama and Hillary Clinton on many of the key issues before this country,” he said on “This Week.”

On “Face the Nation,” Christie fired back: “Marco himself has said that I was a conservative reformer in New Jersey.”