That's what this article in today's Washington Post asks. Judical Watch, a conservative group, argues that, based on surveys conducted by the Border Patrol, that many undocumented immigrants attempting to cross the Mexico-U.S. border were doing so because they believed Bush was offering amnesty (he had offered some very vague proposals about a guestworker program where undocumented immigrants already in the U.S. could receive temporary legal status).
This uninformed suggestion is belied by a recent study from Douglass Massey (released by the CATO Institute, a libertarian think-tank), which shows U.S. immigration policy since NAFTA has actually increased the flow of undocumented immigrants by pursuing economic integration with Mexico while prohibiting the free movement of labor. Massey also argues stepped-up border enforcement not only has not discouraged undocumented immigrants from emigrating to the U.S., but it caused many immigrants to stay in the U.S. longer than they would have because of the difficulties in crossing the border. This argument, which makes perfect sense, should be heeded by those who base their support for more border enforcement on how much it costs taxpayers (incidentally, most immigrants pay taxes also, but I won't get in to that debate right now) to pay for public services used by immigrants and their families.
Blaming amnesty proposals for an increase in "illegal" immigration (Bush did not propose amnesty, nor have any of the other immigration reform bills, particularly the McCain-Kennedy bill) is a common argument by anti-immigrant groups. A much more important cause of undocumented immigration is the large number of jobs requiring cheap labor in the United States.