Substitute \( x = 1 \) into the polynomial \( p(x) = ax^2 - 3(a - 1)x - 1 \):
\[
a(1)^2 - 3(a - 1)(1) - 1 = 0.
\]
Simplify:
\[
a - 3(a - 1) - 1 = 0 \quad \Rightarrow \quad a - 3a + 3 - 1 = 0 \quad \Rightarrow \quad -2a + 2 = 0 \quad \Rightarrow \quad a = 1.
\]
Thus, the value of \( a \) is \( \boxed{1} \).