WASHINGTON — Two leading Republican presidential hopefuls waded into the argument over childhood vaccinations Monday, with Sen. Rand Paul declaring that he had heard of “many tragic cases” of children suffering harm after receiving shots and New Jersey Gov. Chris Christie saying parents should be given a choice on the issue.

The remarks, coupled with President Barack Obama’s defense of vaccinations over the weekend, injected an unexpectedly partisan element into a policy issue — how readily to give exemptions to parents who don’t want vaccines for their children — that until now had not shown much partisan division.

Christie, who has courted controversy with actions on public health, seemed to waver on the issue during the course of the day. He first made a statement that seemed to side, at least in some cases, with parents who want exemptions, and then issued a clarification after coming under attack by Democrats.

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Paul, of Kentucky, stuck more consistently to his libertarian views. On a conservative radio program Monday morning, he said he favored vaccines but that “most of them ought to be voluntary.”

Later, in an interview on CNBC, he upped the stakes, repeating that he saw the question as an “issue of freedom,” then appearing to side with vaccine critics who have linked the shots to autism or other mental problems — contentions that have been repeatedly debunked and, in the case of autism, shown to be based on fraudulent research.

“I think vaccines are one of the greatest medical breakthroughs that we have. I’m a big fan,” Paul said. But then he pivoted to criticism of government vaccination requirements.

“I’ve heard of many tragic cases of walking, talking, normal children who wound up with profound mental disorders after vaccines,” he said. “I think the parents should have some input. The state doesn’t own your children. Parents own the children, and it is an issue of freedom.”

Paul’s comments drew criticism from Democrats and some within the GOP.

“This is not a complex issue,” Republican consultant Matt Mackowiak said. “It’s a public health issue; it’s not a liberty issue.”

Christie’s remarks began with a reporter’s question in London, where the governor was visiting a pharmaceutical company, part of an overseas trade mission that also serves as a way to buff up the governor’s foreign policy credentials for his widely expected presidential bid.

“There’s a debate going on right now in the United States, the measles outbreak that’s been caused in part by people not vaccinating their kids,” the reporter noted. “Do you think Americans should vaccinate their kids? Is the measles vaccine safe?” he asked.

Christie responded that he and his wife had gotten their four children vaccinated. “That’s the best expression I can give you of my opinion,” he said. “But I also understand that parents need to have some measure of choice in things as well, so that’s the balance that the government has to decide.

“It depends on what the vaccine is, what the disease type is and all the rest,” he said. “You have to have that balance in considering parental concerns because no parent cares about anything more than they care about protecting their own child’s health.

“Not every vaccine is created equal and not every disease type is as great a public health threat as others,” he said.

Christie never responded directly to the question about measles and left unclear which vaccines he was referring to. New Jersey has more stringent vaccination requirements than many other states. It is one of only a few states, for example, that require children to get the flu vaccine to attend preschool.

In 2009, when Christie first ran for governor, he questioned the flu requirement and wrote a letter in which he pledged to “stand with” parents who had “expressed their concern over New Jersey’s highest-in-the-nation vaccine mandates.”

On Monday, Democrats quickly went on the attack, with the Democratic National Committee issuing a statement accusing Christie of pandering to the “radical, conspiracy-theory base” of the Republican Party.

The statement contrasted Christie’s remarks with those of Obama, who had answered a question about the issue in an interview over the weekend with NBC’s Savannah Guthrie.

“I understand that there are families that, in some cases, are concerned about the effect of vaccinations. The science is pretty indisputable. We’ve looked at this again and again,” Obama said.

“There is every reason to get vaccinated. There aren’t reasons to not get vaccinated,” he said. “You should get your kids vaccinated.”

Christie’s aides quickly issued a clarification.

“The governor believes vaccines are an important public health protection and with a disease like measles there is no question kids should be vaccinated,” the governor’s office said in a statement. “At the same time, different states require different degrees of vaccination, which is why he was calling for balance in which ones government should mandate.”