Republican vice-presidential nominee Mike Pence. Thomson Reuters

The Seventh Circuit Court of Appeals struck down Mike Pence's policy on refugee resettlement on Monday, at one point calling the Indiana governor's arguments "nightmare speculation."

"The governor of Indiana believes, though without evidence, that some of these persons were sent to Syria by ISIS to engage in terrorism and now wish to infiltrate the United States in order to commit terrorist acts here," the decision by a panel of three judges said.

It continued: "No evidence of this belief has been presented, however; it is nightmare speculation."

For Pence, Republican presidential nominee Donald Trump's running mate, the decision was handed down the day before he is set to debate Democratic vice-presidential nominee and US Sen. Tim Kaine in Farmville, Virginia.

Pence has been a vocal critic of President Barack Obama's initiative to resettle 10,000 Syrian refugees in the US this year, citing security concerns and the belief that ISIS operatives can sneak into the US under the guise of refugees and commit terrorism on US soil.

Pence's stance against refugee resettlement reached its apotheosis after the Paris terrorist attacks in November.

After those attacks, he announced that Indiana would not accept any incoming Syrian refugees and directed state agencies not to allocate federal funds toward refugee resettlement. His announcement came at about the same time Trump proposed a temporary ban on Muslim immigration into the US.

Syrian refugees. Alexander Koerner/Getty Images

Exodus Refugee Immigration, an organization that works with refugees to resettle them in the US, sued Pence in conjunction with the American Civil Liberties Union on the grounds that Pence did not have the constitutional right to bar refugees from Indiana.

In February, the court ruled in favor of Exodus and the ACLU, with Judge Tanya Walton Pratt saying Pence's directive "in no way" furthered the government's "asserted interest in the safety of Indiana residents."

Pence's administration appealed the decision, and a hearing was set for September 14 at the Seventh Circuit Court of Appeals. In its decision Monday, the court unanimously upheld the lower court's ruling, emphasizing the stringency of the refugee vetting process while acknowledging "there can be no certainty that no terrorist will ever slip through the screen."