Any idea why the SSL certificate for that site would be invalid? I'd rather not connect to a site, given the nature of the content, if I can't know who maintains the site and where a valid SSL certificate is a good first step in due diligence.

I'm getting a valid certificate so maybe you are being MITM'd?

I'm getting a valid cert.

http://i.imgur.com/FQ4Yb94.png

If/when this happens again, run this command and paste the output:

    $ openssl s_client -showcerts -connect rally.stopwatching.us:443

Why would you care about an invalid cert if you aren't handing over any credentials?

An invalid cert could indicate a MITM attack. Even if you're not sending anything down, who knows what they're sending back?

Because someone might alter the content, eg: changing the date of the rally (so some people don't turn up on the right day), or making the event much look more unconvincing so you don't bother going.

I still don't understand how a rally will do anything beyond making the news. The effort might be better spent identifying specific senators, and representatives, and encouraging people to vote them out of office through campaigning.

People aren't going to vote for a congress member based on an issue they don't care about. You have to get people to care first, that's what the rally and associated media attention are for.