What is harmful for electronic circuits? Voltage or electrical current?
A device draws the correct current only at the correct voltage (usually specified as a range). If you change the voltage (go outside the specified range), then the current that it draws might change too, to an unsafe value.

Here are a few examples:


With resistors, current rises linearly with voltage. Power dissipation rises with the square of the voltage. Usually not too much of a problem, at least until you reach the dissipation limits.
With capacitors &mdash; especially electrolytics &mdash; there is a voltage above which the current rises dramatically when the internal insulation breaks down.
With transformers, raising the voltage raises the current which increases the magnetic field within the core. When the core reaches saturation, the current rises dramatically, leading to excessive dissipation in the windings.