According to the Electrophoretic Theory, the voltage will increase.
During electrophoresis, one of the parameters is held constant and the other two are allowed to vary as the resistance of the electrophoretic system changes.
In a vertical system when wattage is held constant, the velocity of the samples will decrease because the current, which is in part carried by the DNA, decreases to compensate for the increase in voltage.
When the current is held constant, the samples will migrate at a constant rate. Voltage and wattage will increase as the resistance increases, resulting in an increase in heat generation during the run.
When voltage is set constant, current and wattage will decrease as the resistance increases, resulting in a decrease of heat and DNA migration.
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