put (one's) thumb on the scale

To manipulate a situation in order to create an unfair advantage by deceiving others. Likened to a merchant holding a thumb on the scale when weighing goods for sale, therefore increasing the apparent weight and price. You have to suspect that the casinos put their thumb on the scale when it comes to the slot machines. There's no way you're getting fair odds. These giant corporations are constantly putting their thumbs on the scale, using their vast resources to influence public policy in their favor.