The Commerce Clause grants Congress the power to regulate local business activity if any part of it affects interstate commerce, if the aggregate of activity of that industry has a substantial effect on interstate commerce.
Congress acted within its power to protect interstate commerce in extending coverage of Title II of the Civil Rights Act to restaurants serving food moving in interstate commerce. Congress had had ample basis to conclude that discrimination based on race by such restaurants burdens interstate trade.
In Heart of Atlanta Motel v. United States, the Supreme Court found that the Act was a valid exercise of Congress’s power to regulate interstate commerce in prohibiting hotels from discriminating against customer based on race.
http://www.lawnix.com/cases/katzenbach-mcclung.html
Under current Supreme Court thinking they will not be able to use the Commerce Clause to regulate neighborhood watch groups.
I think the only theory they can pursue would be the neighborhood watch group as some sort of de facto state actor because they in fact work so closely with de jure law enforcement.
Under the latter theory, Congress could regulate directly under the Fourteenth Amendment as authority.
I don’t agree that is legally proper, but I could see lawless liberal judges relying on that.