The constitution requires that the executive "shall take Care that the Laws be faithfully executed." The executive is not granted unlimited power to refuse to enforce the law.

However, courts have historically given the executive leeway in how it chooses to prioritize things and this has led to plenty of cases where the federal government has official policy to deprioritize enforcement of certain crimes. This has not been abused too much in the past but I'm not sure if that norm will hold in the future.