How can the commerce clause expand the power of the national government?
The Commerce Clause can expand the power of the national government because almost anything can be seen as interstate commerce if the courts are willing to interpret that in a broad way. The courts have generally been willing to do so and the federal government has used the commerce clause to regulate a number of areas that might seem to be the responsibility of the states.
As one example of this, the federal government uses the commerce clause to justify its bans on discrimination and segregation in public facilities. The federal government is allowed to ban segregation in restaurants, for example, because the food sold in the restaurants might be part of interstate commerce. The customers in the restaurants could also be part of interstate commerce.
Interstate commerce, then, has been defined very broadly by the courts. This has allowed the federal government to enact regulations in many areas that might not seem to be part of interstate commerce.