FILE – In this Aug. 19, 2013 file photo, Sen. Mark Kirk, R-Ill. speaks in Chicago. Kirk is leading a group of Republican senators who are pressing administration officials to halt Syrian refugee admissions over concerns that terrorists could exploit fake Syrian passports to enter the United States. (AP Photo/M. Spencer Green, File)

A group of Republican senators led by Sen. Mark Kirk (R-Ill.) is pushing the Obama administration to put in place a Syrian refugee moratorium over fears that terrorists might use fake passports to enter the United States.

In a letter sent on Wednesday to Secretary of State John Kerry and Homeland Security Secretary Jeh Johnson, the group of seven Republican senators said the administration should “pause” Syrian refugee admissions until they can come up with “a credible plan to detect and identify all fake Syrian passports and to prevent ISIS terrorist infiltration.”

“Anything less represents an unacceptable risk to the safety of our country,” reads the letter, which was also signed by Republican Sens. Bill Cassidy (la.), John Barrasso (Wyo.), Mike Enzi (Wyo.), Thom Tillis (N.C.), Roger Wicker (Miss.) and James M. Inhofe (Okla.).

Last month, Senate Democrats blocked a House-passed bill that would prevent Syrian and Iraqi refugee admissions until the FBI director, Homeland Security secretary and Director of National Intelligence could vouch that every admitted refugee would not pose a security threat. The White House opposed the bill and President Obama has condemned the idea of blocking certain refugees from coming to the United States.

But for Kirk, the issue has became a flash point in his re-election campaign, where he is in a tight contest with Rep. Tammy Duckworth (D-Ill.).

In September, Duckworth signed onto a letter urging President Barack Obama to take in 200,000 refugees, including 100,000 Syrians, by the end of 2016. In the wake of the terror attacks in Paris, Kirk seized on Duckworth’s appeal, arguing in a November op-ed in the Chicago Tribune that the United States had “already let terrorists in as refugees.” He launched an attack ad against Duckworth on the issue in December.

In their letter, the senators highlighted concerns circulating since the November Paris terror attacks after a fake Syrian passport was found on the body of one of the attackers. The stamps in it suggested he had entered Europe through the trail commonly followed by refugees.

Syrian passports have also become a hot commodity for other refugees simply seeking safe haven, as some countries have announced special plans to take in additional Syrian refugees due to that country’s worsening war. Last year, the Obama administration announced plans to take in an additional 10,000 Syrian refugees – far less than the millions streaming into Europe.

Syrian refugees currently go through a vetting process that takes anywhere from 18 to 24 months. Both advocates and refugee resettlement agencies argue it is one of the most comprehensive and thorough immigration procedures the United States conducts. But last year, FBI director James Comey told lawmakers that the realities on the ground — much of Syria is in the throes of a bloody conflict — made it difficult to conduct thorough background checks on would-be refugees.

Lawmakers pushing the moratorium legislation have often cited Comey’s testimony as evidence that the program needs an overhaul. Comey, however, never supported the legislative effort to curtail the refugee resettlement program.