The question arises because the U.S. Constitution did not give Congress the power to legislate generally, over any subject, but only over the subjects listed in the Constitution, one of which is the regulation of interstate commerce.
And so, in S. 909, the "Matthew Shepard and and James Byrd Jr. Hate Crimes Prevention Act," which yesterday became part of the 2010 defense appropriate act (S. 1390), Congress dutifully found that violence motivated by bias "affects interstate commerce."
When the Constitution was first proposed and then ratified in the late 1700s, interstate commerce was relatively limited. Most food was grown within a few miles of where it was eaten, and most goods were manufactured locally, and by hand. The industrial revolution was just beginning, and the only means of transporting goods long distances was by horse-drawn wagon or by sea.
As our economy has grown larger and more complicated, the importance of interstate commerce has also grown. Today, it's difficult to find anything in any store that was not either grown or manufactured in a different state or includes materials from a different state. And as interstate commerce grew, the power of Congress grew, so that Congress began regulating not just railroads and the interstate movement of goods, but also agriculture, manufacturing, working conditions, and product safety.
When it involves interstate commerce, Congress can legislate against discrimination and bias. The Civil Rights Act of 1965, which made it illegal for hotels, restaurants, and other public accommodations to discriminate based on race was based on the power of Congress to regulate interstate commerce, because there is no power in Congress to legislate against discrimination generally. (The 14th Amendment prohibits states from denying equal protection and does not prohibit private discrimination.) Later legislation has prohibited discrimination in housing and employment, and has extended to not just racial discrimination but also discrimination based on gender, ethnicity, religion, and disabilities.
I agree with all of this. We have a national economy, and we should have national laws regulating that economy. But do two homophobes beating up a gay man outside of a bar really affect the national economy?
According to Congress, it does. Section 2 of S. 909 states (in part) that violence motivated by bias:
substantially affects interstate commerce in many ways, including the following:
(A) The movement of members of targeted groups is impeded, and members of such groups are forced to move across State lines to escape the incidence or risk of such violence.
(B) Members of targeted groups are prevented from purchasing goods and services, obtaining or sustaining employment, or participating in other commercial activity.
(C) Perpetrators cross State lines to commit such violence.
(D) Channels, facilities, and instrumentalities of interstate commerce are used to facilitate the commission of such violence.
(E) Such violence is committed using articles that have traveled in interstate commerce.
The statute itself limits the crimes relating to gender, religious, gender identity, and other biases to those occurring "during the course of, or as the result of, the travel of the defendant or the victim--(I) across a State line or national border; or (II) using a channel, facility, or instrumentality of interstate or foreign commerce," those committed using a "channel, facility, or instrumentality of interstate or foreign commerce" or using a "firearm, dangerous weapon, explosive or incendiary device, or other weapon that has traveled in interstate or foreign commerce," and those that "interferes with commercial or other economic activity in which the victim is engaged at the time of the conduct" or "otherwise affects interstate or foreign commerce."
It's difficult to imagine any crime of violence, anywhere, that wouldn't fall under one of those categories. For example, it's pretty safe to say that guns, knives, and even baseball bats are manufactured and sold in interstate commerce, so any crime, committed with any gun, knife, or baseball bat that ever crossed any state line, can now be a federal crime.
And my hypothetical about two homophobes beating up a gay man outside of a bar is a crime falling within the new statute if the fight arose after the victim bought a beer in the bar (which is interstate commerce) or caused the victim to miss a day of work (which affects interstate commerce).
United States v. Lopez, 514 U.S. 549 (1995), is often cited as limiting Congressional power, because in that case the Supreme Court ruled unconstitutional a law that made it a crime to carry a gun within a "school zone," rejecting arguments raised by the government that violence near schools will affect interstate commerce. However, the statute in question did not include any specific requirement that the crime be found to be "in interstate commerce" or "affect interstate commerce," and there were no findings by Congress about how the presence of guns near schools would affect interstate commerce, so the decision could be distinguished from S. 909 and it is not clear how the Supreme Court will react to "findings" and statutory "limits" like those found in S. 909.
One would think that there is a limit to congressional power, and that Congress cannot extend its power merely because the crime is carried out using some weapon that once crossed a state line, or because of some relatively minor and unintended economic consequence of the crime. But maybe not.