Will Wilkinson on similarities between the Wisconsin protests and the Tea Party protests:

There’s something about the union demonstrations in Madison, and the excitement it has caused on the left, that reminds me of the Tea Party. I think I’ve figured it out what it is. The advent of the labor movement is at the heart of the left’s sacred creation myth. The sense on the left that unions are under siege gives them something to fight for with a bracing sense of historically-rooted identity and moral authority. Similarly, the sense on the right that America’s foundational values are under siege gave the Tea Party something to fight for with a bracing sense of historically-rooted identity and moral authority.

I think the commonality is loss aversion. Tea Party protesters were motivated by the sense that money was being taken from them and given to "the losers." Wisconsin protesters are rallying because their right to bargain collectively for better wages and working conditions is being threatened. It's just much easier to motivate somebody to protest against a perceived threat to their livelihood than in favor of a positive change. It's not a surprise that a bread-and-butter Republican power grab is what's motivated the most passionate grassroots movement of the Obama presidency.