Take, as the sample space, the set S of all voters. The events are as follows:

E: the event that a voter votes for the Republican candidate; P(E) = 0.30
F: the event that a voter votes for the Democrat candidate; P(F) = 0.35

The experimental probability that a voter will vote for a presidential candidate of either major political party is then P(EF). Since these are mutually exclusive events (it is impossible to vote for both candidates),

P(EF) = P(E) + P(F) = 0.30 + 0.35 = 0.65.