That's a very interesting idea. One obvious downside is a congressman might change their position faster than you can because they have advanced knowledge, so it would be riskier to hold the same position. Especially if that linked page updates even a little slowly. Secondly, they might have that position knowing exactly that - that they will have advanced knowledge and there it's worth having it at all. They know they have advanced knowledge so they are capable of riskier things. They also know that people can see their positions so they could game everyone by luring them into positions which they could exit quickly so as to pump and dump.

Is there an ETF yet?