Sure, the terror attacks in Brussels are likely to make a difference to current public opinion regarding the Brexit vote[a][b][c][d]. But will they make much difference come June 23? There is some evidence that suffering a terrorist attack can have long-term affects on voters[e]. Terrorism may increase voter turnout levels[f], and there’s some evidence that it causes voters to skew right a bit[g]. Moreover, the 2004 attack in Madrid seems to have had an impact on the election there, helping to cause Spain to disentangle itself from commitments that made it more vulnerable to terrorism[h]. However, that attack took place a mere three days before voters went to the polls.
But I doubt that Brussels is a game changer for Brexit. All of these studies focus on cases in which the polity has been a direct victim of terrorism, and an attack on Brussels is not an attack on London, at least as far as the average British voter is concerned. Moreover, the terrorist attacks in Brussels didn’t teach anyone anything fundamentally new. Without much evidence, I suspect that British voters already had a pretty good idea of what they might be in store for after November 13.
I’ll keep an open mind, but right now I can’t see the recent events in Brussels having much of an affect on the Brexit vote: 61% Britain remains in the EU.