WASHINGTON » Republican presidential rivals rushed today to condemn Donald Trump’s support for a government database to track Muslims in the United States, drawing a sharp distinction with the Republican front-runner on a proposal also deemed unconstitutional by legal experts.

Former Florida Gov. Jeb Bush called the prospect of a registry “abhorrent.” Florida Sen. Marco Rubio said the idea was “unnecessary” and not something Americans would support. And Texas Sen. Ted Cruz, who has largely avoided criticizing Trump throughout the 2016 campaign, said, “I’m not a fan of government registries of American citizens.”

“The First Amendment protects religious liberty, and I’ve spent the past several decades defending the religious liberty of every American,” Cruz told reporters in Sioux City, Iowa.

The rebukes came after Trump voiced support for a mandatory database for Muslims in the U.S. while campaigning Thursday in Iowa. The real estate mogul was asked by an NBC News reporter about the prospect of a database and whether Muslims would be required to be registered. In a video posted by the network, Trump said, “They have to be.”

Asked whether Muslims would have to register at mosques, Trump said: “Different places. You sign up at different places. But it’s all about management.”

On Friday, Trump said on Twitter that he didn’t suggest creating a database but instead was answering a question from a reporter. However, he did not disavow the prospect of a registry on social media or at an event Friday morning.

Trump has also voiced support for closing certain mosques as a way to contain the terror threat in the U.S.

His comments followed the attacks in Paris that killed 130 people and wounded hundreds more. The Islamic State group has claimed responsibility, elevating fears in the U.S. and prompting calls for new restrictions on refugees fleeing war-torn Syria.

The House passed legislation this week essentially barring Syrian and Iraqi refugees from the United States. Senate Majority Leader Mitch McConnell, R-Ky., has slotted the bill for possible Senate consideration, though it’s unclear whether the chamber could get enough votes to override a threatened veto by President Barack Obama.

The Republican candidates’ unified criticism of Trump was striking. His rivals have vacillated in their handling of other inflammatory comments from him, wary of alienating his supporters while increasingly concerned that he’s maintained his grip on the GOP race deep into the fall.

Civil liberties experts said a database for Muslims would be unconstitutional on several counts, while the libertarian Cato Institute’s Ilya Shapiro said the idea also violates basic privacy and liberty rights.

Marci Hamilton, a Yeshiva University legal expert on religious liberty, said requiring Muslims to register appears to be a clear violation of the Constitution’s protection of religious freedom.

“What the First Amendment does and what it should do is drive the government to use neutral criteria,” Hamilton said. “You can use neutral criteria to identify terrorists. What it can’t do is engage in one-religion bashing. That won’t fly in any court.”

Meanwhile, the Anti-Defamation League in New York called Trump’s proposal “deeply troubling and reminiscent of darker days in American history when others were singled out for scapegoating.”

Democratic front-runner Hillary Rodham Clinton slammed Trump during a rally in Nashville Friday evening. “Mr. Trump has attacked Mexican immigrants, he’s attacked women, and now he’s attacking Muslim Americans,” she said. “At some point you have to ask yourself, is that the kind of country we are?”

Vermont Sen. Bernie Sanders called Trump’s words “outrageous and bigoted.”

Back on the GOP side, New Jersey Gov. Chris Christie said creating a national registry based on religion and closing mosques “will do nothing to keep us safer and shows a lack of understanding on how to effectively prevent terrorist attacks.”

Ohio Gov. John Kasich said requiring people to register with the federal government because of their religion “strikes against all that we have believed in our nation’s history.” Kasich had faced criticism following the Paris shooting for saying he would set up an agency with a mandate to promote what he called “Judeo-Christian values” overseas to counter Islamist propaganda.

Ben Carson, the retired neurosurgeon who has challenged Trump’s lead in the GOP primary, has also made eyebrow-raising statements about Muslims following the Paris attacks.

“If there’s a rabid dog running around in your neighborhood, you’re probably not going to assume something good about that dog,” Carson said Thursday. “It doesn’t mean you hate all dogs, but you’re putting your intellect into motion.”

Asked Friday about a registry for Muslims, Carson said the U.S. should have a database on “every foreigner who comes into this country,” but he rejected the idea of tracking U.S. citizens based on their religion.

“One of the hallmarks of America is that we treat everybody the same,” he said. “If we’re just going to pick out a particular group of people based on their religion, based on their race, based on some other thing, that’s setting a pretty dangerous precedent.”

The first reference to a database came in a Trump interview with Yahoo News published Thursday. There, he did not reject the idea of requiring Muslims to register with the government or giving them special identification cards noting their religion.

“We’re going to have to look at a lot of things very closely,” Trump told Yahoo.

Trump was pressed on the database by NBC Thursday evening. Later that night, Trump told reporters he “never responded” to the questions from Yahoo and ignored follow-ups about his remarks to NBC.

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Colvin reported from Spartanburg, South Carolina. Associated Press writers Bill Barrow in Mobile, Alabama; Steve Peoples in Sioux City, Iowa; Catherine Lucey in Des Moines and Julie Bykowicz and Mark Sherman in Washington contributed to this report.