Most Democratic leaders, many Republican leaders, and president Bush all support an amnesty for illegal aliens, and that amnesty would indeed strongly increase the likelihood of another 9/11.

Chapter 3 of the 9/11 Commission Staff Report (Terrorist Entry and Embedding Tactics, 1993 to 2001), among many other things, contains this: Mahmoud Abouhalima, involved in both the World Trade Center and landmarks plots, received temporary residence under the Seasonal Agricultural Workers (SAW) program, after falsely claiming that he picked beans in Florida.

And, from this: A draft government report shows the agency that would oversee any future guest-worker program doesn't have a handle on fraud, doesn't do enough to deter it, and won't have a fraud-management system in place until 2011 -- years after its proponents want a program to start.

And, from this : An estimated 75 percent of applicants for immigrant benefits - green cards, work visas, and a host of other documents - at a major federal processing center were not screened through the U.S. terrorism watch list over the past four years, the Daily Bulletin has learned. (The whistleblower in that case has since had his contract not renewed...)

And, there's also this new House report (PDF file) that contains, among much other information, this: During 2005, Border Patrol apprehended approximately 1.2 million illegal aliens; of those 165,000 were from countries other than Mexico. Of the non-Mexican aliens, approximately 650 were from special interest countries. Special interest countries are those "designated by the intelligence community as countries that could export individuals that could bring harm to our country in the way of terrorism." ...Members of Hezbollah, the Lebanon-based terrorist organization, have already entered to the United States across our Southwest border. On March 1, 2005, Mahmoud Youssef Kourani pleaded guilty to providing material support to Hezbollah. Kourani is an illegal alien who had been smuggled across the U.S.-Mexico border after bribing a Mexican consular official in Beirut for a visa to travel to Mexico. Kourani and a Middle Eastern traveling partner then paid coyotes in Mexico to guide them into the United States. Kourani established residence among the Lebanese expatriate community in Dearborn, Michigan and began soliciting funds for Hezbollah terrorists back home in Lebanon. He is the brother of the Hezbollah chief of military operations in southern Lebanon...

So, once you look at all the information that's out there, it's pretty obvious that anything like "comprehensive" immigration reform (aka a massive amnesty) would greatly increase the risk of terrorist attacks in the U.S.