>Are you aware of any such corpus?

I am not aware of a corpus for political sentiment specifically. There's a general twitter sentiment dataset[0], the link appears to be broken but it's what everyone cites, not sure why it's down.

This paper[1] uses tweets and emoticons in the tweet as a soft label for sentiment, there's obvious issues with that, but it's a cheap way to get lots of noisy labels.

[0]: http://www.sananalytics.com/lab/twitter-sentiment/

[1]: https://www.aaai.org/ocs/index.php/SSS/SSS13/paper/download/...