Sen. Marco Rubio isn't jumping on Donald Trump's birthright bandwagon.

The Florida Republican, a leading presidential candidate, told Fox News' Bill O'Reilly on Thursday that he opposes repealing birthright citizenship. Doing so, by constitutional amendment if necessary, is a key plank of GOP 2016 contender Trump's immigration policy proposal.

The legal consensus is that the Fourteenth Amendment to the Constitution, passed in the wake of the Civil War, bestows, with some exception, U.S. citizenship to virtually any child born on U.S. soil. That includes children born to parents who are living in the U.S. illegally.

Rubio's position separates him from some of his competitors for the Republican nomination. A few have rushed to tout their opposition to birthright citizenship (Sen. Ted Cruz of Texas), or pointedly declined to take a position (Gov. Scott Walker of Wisconsin) in the days since the New York billionaire businessman and reality television star, leading in most polls, publicized his opposition to this constitutional right as part of his broader plan to reduce both legal and illegal immigration.

"You're talking about the 14th Amendment, I do not support repealing it," Rubio told O'Reilly, when the broadcaster asked the senator to address the issue. "No. 1, I don't think we can; and No. 2 while there is some interesting debate going on, as I think you pointed out in your earlier segment, about jurisdiction thereof, that whole legalistic question, the prevailing belief is that, in fact, it says that anyone born in the territory of the United States, irrespective of the status of their parents, unless they're diplomats, are U.S. citizens. That's not going to change, and I don't support changing that."

Rubio conceded that the law has been abused. In the course of his discussion with O'Reilly, he discussed abuses he has witnessed first hand in his home town of Miami and suggested he's open to public policy that would discourage foreign parents from coming to the U.S. — legally or illegally — for the sole purpose of giving birth to a child so that that child can obtain citizenship.

"I see it in South Florida where I live. I'm not talking about poor people. These are wealthy families that come in, eight and a half months pregnant, from Latin America, they go to the hospital, they have a child with U.S. citizenship, they go back to their country of origin and they're hedging their bets that if things ever go wrong, they have citizenship," Rubio said. "I have said I am open to exploring ways to looking at people that are deliberately coming here for purposes of having a child. I don't know how you do that."

Rubio, 44, is the son of parents who were legal immigrants from Cuba but not yet citizens when he was born. He previously supported comprehensive immigration reform that included a path to citizenship for some illegal immigrants, but now backs a step-by-step process that addresses border security and interior enforcement first. Once Americans are confident that the border is secure, Rubio would address the 11-12 million illegal immigrants currently residing in the U.S.

In an interview Friday, Rubio campaign spokesman Alex Conant elaborated on the senator's support for birthright citizenship and willingness to crack down on abuses of the law. Conant said the two positions are consistent, likening it to support for the First Amendment while at the same time backing laws guarding against the abuse of free speech. As for specific proposals, Rubio doesn't have anything in mind right now.

"He's open to ideas," Conant said.

Heading into the weekend, Rubio was running fifth in the field of 17 Republican presidential candidates, with 7.3 percent support, according to the average national polls gauging primary voters' preferences. Trump was running first with 22 percent. The New Yorker introduced his immigration plan, and it dominated the week, as reporters buttonholed candidates on the trail and asked them what they thought, particularly about the more controversial elements.

In addition to wanting to end birthright citizenship, Trump has proposed forcibly rounding up and deporting the 11-12 million illegal immigrants; leveling tariffs on Mexico if they refuse to pay the U.S. to construct a security wall along the southern border; forcibly rounding up millions of illegal immigrants, even deporting their citizen family members if necessary; and searching through and impounding financial remittances that illegal immigrants send to family back in their home countries.

On Wednesday, Trump during an interview with O'Reilly dismissed criticism that his immigration plan is a political liability for the Republican Party in the 2016 elections.

"We have at least 11 million illegals in the country; not only the jobs they're taking but everything else. And you know about the crime wave," Trump said. "We have to do something about it." On the birthright citizenship, he added: "What happens is, they're in Mexico, they're going to have a baby, they move over here for a couple of days, they have the baby … When people are illegally in the country they have to go."

Disclosure: The author's wife works as an adviser to Scott Walker.