Nearly two dozen Republican governors moved Monday to close off their states to Syrian migrants as leading GOP presidential candidates outlined positions that would discriminate against Muslims seeking refuge in the United States.

The efforts come amid heightened fears among Republicans, and some Democrats, that potential terrorists may be concealing themselves among the tens of thousands of people fleeing Syria’s civil war. At least one person who carried out Friday’s Paris attacks, which left 129 dead, appears to have entered Europe on a Syrian passport as part of the wave of Middle Eastern migrants arriving in Greece.

President Obama, who has been criticized sharply for what some describe as a feckless Syria policy, has allowed for 10,000 Syrian refugees to resettle in the United States over the next year. But a growing number of prominent Republicans, including several seeking the presidency, have recommended accepting only Christian migrants in the United States and excluding all Muslims.

At a news conference Monday in Antalya, Turkey, where Obama was attending the Group of 20 summit, the president denounced those proposals as contrary to U.S. humanitarian values, which he said must be upheld to strengthen American influence in the world.

“When I hear political leaders suggesting that there would be a religious test,” Obama said, “that’s shameful. That’s not American. That’s not who we are. We don’t have religious tests to our compassion.”

Republican presidential candidate Chris Christie of New Jersey was among at least two dozen governors who declared a halt to accepting Syrian refugees Monday amid security concerns. (Joe Raedle/Getty Images)

The rising unease among many Republicans toward Muslim migrants — and at times the Islamic faith in general — marks a stark and sudden shift in the 2016 campaign to issues of religion and national security. The debate could have significant implications for next year’s general election, in which GOP leaders had hoped to better perform with nonwhite voters than in past contests.

The list of governors proclaiming their refusal to accept Syrian migrants had climbed to at least 23 by late Monday, including presidential candidates Chris Christie of New Jersey, Bobby Jindal of Louisiana and John Kasich of Ohio. The only Democrat joining in was New Hampshire Gov. Maggie Hassan, who is challenging Sen. Kelly Ayotte (R-N.H.) for her Senate seat.

Several governors acknowledged that they do not have the ability to stop the federal government from accepting and financing the resettlement of refugees. Nonprofit agencies that work with the federal government on resettlement said that while the cooperation of states and localities helps in the process, no governor can impede the movement of refugees once they have legal status.

Lavinia Limón, president and chief executive of the U.S. Committee for Refugees and Immigrants, said she is surprised that the once-nonpartisan cause of helping refugees fleeing violence has become so politicized.

“The definition of a refugee is someone fleeing oppression. They’re fleeing terrorism,” she said. “They’ve experienced what happened in Paris on a daily basis.”

Republican strategists are divided over the lasting consequences of the refu­gee issue, with some expecting grass-roots voters to rally to the party’s increasingly aggressive tone and turn out in droves in the upcoming primary contests. Others are nervous about damaging the party’s brand with controversial comments, such as Donald Trump’s remark Monday that he would consider shutting down some mosques and former Arkansas governor Mike Huckabee’s call to “wake up and smell the falafel.”

Gov. Rick Snyder of Michigan and many other U.S. governors are threatening to halt efforts to bring Syrian refugees to their states in the aftermath of the attacks in Paris, though an immigration expert says they have no legal authority to do so. (Paul Sancya/AP)

“It’s very worrisome,” said Ari Fleischer, a former George W. Bush adviser who has worked with the Republican National Committee on expanding the GOP’s reach. “We must always make the distinction between law-abiding Muslims, of which there are many, and radical Islamists who want to do us great harm.”

The two GOP front-runners, Trump and Ben Carson, have both opposed welcoming Muslim refugees and have been cheered by conservative activists for doing so. Their approach is being echoed rather than countered by rivals, indicating that most GOP hopefuls believe that the party’s base supports that view.

Trump on Monday urged federal law enforcement officials to implement heavy surveillance of mosques and said he would consider closing Islamic places of worship as president.

“I would hate to do it, but it’s something that you’re going to have to strongly consider because some of the ideas and some of the hatred — the absolute hatred — is coming from these areas,” Trump said on MSNBC’s “Morning Joe.”

At a news conference Monday in Nevada, Carson said that the United States “cannot, should not and must not accept any Syrian refugees.” He also said that an “ideological test” would be appropriate and that Syrians currently living in the United States should be closely monitored.

Sen. Rand Paul (R-Ky.), another 2016 candidate, announced new legislation Monday that would deny entry to anyone from up to 30 nations with “jihadist elements.” That would include refugees from Syria, as well as a 30-day waiting period for French citizens.

“It’s about time, and Paris should be a wake-up call,” Paul told reporters on a conference call. “. . . No one’s immigrating, no one’s studying, no one’s entering our country with special permission.”

A pair of Republican candidates from opposite wings of the party — Sen. Ted Cruz (Tex.) and former Florida governor Jeb Bush — voiced similar trepidation about bringing in Muslim refugees from Syria and suggested that only Christians from the region should be considered for entry.

When Cruz was asked Monday about Obama’s objection to this GOP sentiment, he laughed before answering.

“It is one of the saddest things we’ve seen for seven years, that President Obama has consistently abandoned and alienated our friends and allies and has coddled and appeased our enemies,” Cruz said in Sun City, S.C. He later said he planned to introduce his own legislation to prevent Syrian Muslims from migrating to the United States.

Sen. Lindsey O. Graham (R-S.C.), a hawkish long-shot candidate who has cautioned his party to not lurch right on immigration, called for a “timeout on Syrian refugees” in an interview with Fox News Radio. Christie, a blue-state Republican, told radio host Hugh Hewitt that he would not even allow “orphans under 5” from Syria to enter.

The question is complicated for many evangelicals, who worry about the plight of Christians in the Middle East but are also often deeply conservative on border and immigration issues.

“This will cross-pressure the evangelical base,” said Brett O’Don­nell, a political consultant and Graham adviser. “There is a tension here in a lot of their circles about ‘Should we support taking any Syrians when it seems like the right thing to do?’ But their position on immigration is so hard-line at the same time.”

On Capitol Hill, some Republicans were pushing ahead with legislation aimed at limiting or halting the influx of refugees from Syria.

House Speaker Paul D. Ryan (R-Wis.) on Monday did not call for blocking refugees but did not rule out the possibility. “We’ve always been a generous nation taking in refugees, but this is a unique situation,” Ryan said in an interview with talk-radio host William J. Bennett.

President Obama made remarks and answered questions at the G-20 summit in Turkey on Nov. 13. Here's what he said about the path forward fighting the Islamic State, welcoming Muslims and protecting Syrian refugees. (AP)

In Turkey, a frustrated Obama warned Republicans not to go too far in embracing the inclination of their party’s base by opposing Syrian refugees and discriminating against Muslims.

“Slamming the door in their faces would be a betrayal of our values,” Obama said. “Our nations can welcome refugees who are desperately seeking safety and ensure our own security. We can and must do both.”

Juliet Eilperin, Katie Zezima, Carol Morello, David Weigel and Michelle Boorstein contributed to this report.