House Democratic leaders are lining up against a GOP proposal to stiffen background checks for Syrian and Iraqi refugees.

Republican leaders are expected to vote Thursday on the measure, which would require new layers of screenings for the refugees fleeing the embattled countries.

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The legislation is a response to last week's terrorist attack in Paris — claimed by the Islamic State in Iraq and Syria (ISIS) — which left at least 129 people dead and hundreds more injured.

Democrats fear the legislation erects barriers so restrictive that it would effectively end the refugee program altogether.

"They make the program unworkable," a Democratic leadership aide said in an email.

"Their bill would immediately shut down refugee resettlement in the Syria and Iraq region and severely handicap refugee resettlement in the future."

The Democrats had offered a revised version which was rejected by the Republicans, the aide added. They are now weighing an alternative bill.

Democratic opposition will likely be inconsequential in this week's House vote, where Republicans are expected to pass the bill with unanimous support. But it presages a tough road in the Senate, where the filibuster gives Democrats more leverage, and may foreshadow a veto threat by President Obama, who has not yet weighed in on the measure.

It would also require the heads of those two agencies, as well as the director of National Intelligence, to certify to Congress that "the covered alien is not a threat to the security of the United States."

As an additional security measure, it also requires the DHS inspector general to review those certifications and provide detailed reports to Congress.

Supporters of the legislation say it's a commonsense strategy for protecting Americans from terrorists who may infiltrate the refugee system. Fueling their argument is the suspicion at least one of the Paris attackers did just that.

“America has a proud tradition of welcoming refugees into our country, and we lead the world in humanitarian assistance," McCaul, the chairman of the House Homeland Security Committee, said Tuesday in introducing his bill. "However, we also must put proper measures in place to ensure our country’s safety.”

Democrats have a sharply different view. While emphasizing Congress' obligation to keep Americans safe with robust screenings, they also want to continue the refugee program for the sake of those fleeing the violence in Syria and Iraq.

If the bill is not enacted as a stand-alone measure, it could resurface next month as a rider to a year-end spending package — a strategy already being pushed by several leading Senate Republicans.