Many states have pretty broad laws in this regard. For example in Minnesota the relevant law reads: "The person driving the motor vehicle shall keep the plate

Many states have pretty broad laws in this regard. For example in Minnesota the relevant law reads: "The person driving the motor vehicle shall keep the plate legible and unobstructed and free from grease, dust, or other blurring material so that the lettering is plainly visible at all times. It is unlawful to cover any assigned letters and numbers or the name of the state of origin of a license plate with any material whatever, including any clear or colorless material that affects the plate's visibility or reflectivity."