Most of the voting is already over by the time he said it. The election is happening in a phased manner and the bulk of seats were polled by 23rd itself.
I'll be frank, the number of actual voters in India who's decision could be swayed by this or any other defence deal is very very low. Most are only concerned about their day-to-day problems, however they do care about scams. But the opinion in public so far is that Supreme Court or CAG did not really find anyone guilty of a scam of the kind Rahul is accusing Modi of.
Plus, the fact that Rahul made this statement AFTER most of the constituencies were already polled shows that even he does not believe his statement will have much of an effect on voters' mind. More than likely this was just to assure the French side of Rafale future in India, regardless of election result.
Maybe even Rahul finally became smart enough to understand the strategic implications?
