I believe Senators have some special immunity, with their own ethics committee dealing with this type of thing. If you were to do the same thing (assuming you're not a Senator! :), trading on inside information could be illegal. As long as it's public information about the Senators investments, it's fine... but isn't that information delayed, making it less useful?
Years ago I saw research that companies which spent more on lobbying had better stock market performance. And I believe Senators manage to gain a lot of wealth during their time in office... so I could believe, all together, that an investment thesis lurks in figuring out what Congress will do next, and with whom. But getting that information when it's very fresh would be quite difficult.