Illusory Truth Effect
TLDR; We tend to believe things that we're exposed to repeatedly
If you've viewed any media at all during election season then you've definitely experienced the repeated campaign ads. They repeat over and over again, often blatant falsehoods about their opponents. This annoyingly repetitive process unfortunately works on people because we have a bias to believe in information that we've been exposed to regularly.
Studies have even shown that this type of repetition works on people who started out knowing that the repeated statement was false. This behavior is in part attributed to "processing fluency" which refers to our ease of processing information. We become more and more efficient at processing information each time it is repeated. This can then lead to false conclusions as we tend to favor the easy to process statements over competing statements.
How to Avoid the Illusory Truth Effect
As with all biases, your awareness of it is your best weapon. When you hear something being repeated ad nauseam, you might be best to change the channel or stop listening altogether. It never hurts to apply some additional scrutiny. Check the facts and make sure you give competing theories a fair shot.