I'm looking for a lower bound for the probability that an arbitrary convex combination of iid Bernoulli (p) random variables is at least p. My guess is p/2k (for some constant k; k must be at least e, as noted by Matt below), but I'm happy with any positive lower bound that depends only on p.
For example, if p is slightly above 1/2, and the convex combination is simply the average of two variables, then the probability is slightly above 1/4 which is approximately p/2.