How Powerful Are Unions Politically?

Nate Silver calculates that the union vote increases the Democratic party's congressional vote by 1.7 to 2.4 points. He warns the GOP:

Republican efforts to decrease the influence of unions while potentially worthwhile to their electoral prospects in the long-term could contribute to a backlash in the near-term, making union members even more likely to vote Democratic and even more likely to turn out. If, for instance, the share of union households voting for Democrats was not 60 percent but closer to 70 percent, Republicans would have difficulty winning presidential elections for a couple of cycles until the number of union voters diminished further.

A new PPP poll measures the political hit that Walker has taken in Wisconsin:

[I]f voters in the state could do it over today they'd support defeated Democratic nominee Tom Barrett over Scott Walker by a a 52-45 margin.