If a television station sold commercial time to a republican candidate for governor but refused to sell time to the democratic candidate for governor, this station would be in violation of the

In that case, the station would be violating equal time rule
equal time rule requires all U.S. radio and television broadcast should provide an equal opportunity for any opposing political candidates that wanted to pay for the air time. This rule does not necessarily specifyy that those stations couldn't show news that favour only a certain candidate.