Yesterday, I wrote about the deal President Obama struck with labor unions to give them a special exemption from the new tax on expensive health benefits until 2017, but I didn’t talk about the political consequences. The problem with Obama’s decision to cater to his big labor allies is that according to the most recent data from the Bureau of Labor Statistics, just 12.4 percent of workers belong to unions (either private sector or public sector ones). And breaking down the numbers to the state level suggests the decision could be particularly harmful, because many states with competitive races have union membership rates in the single digits.

For instance, endangered Sen. Blanche Lincoln will have to run for reelection in Arkansas, a state where just 5.4 percent of the workforce belongs to a union. And in Colorado, where Sen. Michael Bennett will be facing voters in November, only 8.7 percent of workers are unionized.

I couldn’t find reliable, recent data that breaks the numbers down by Congressional district, but to give you an idea, there are 18 Democratic seats considered competitive by the Cook Political Report that are in states where union membership is in the single digits. That includes races in Arizona (8.8%), Alabama (9.5%), Arkansas (5.4%), Colorado (8.7%), Florida (5.9%), Idaho (5.3%), Kansas (7%), Mississippi (6.7%), New Hampshire (9.7%), New Mexico (7.7%), North Carolina (3%), South Carolina (4.1%), Tennessee (5.3%), Texas (4.7%), and Virginia (3.7%).

Obviously, this is a rough analysis, and there are a lot of other factors that will affect outcomes in each individual race. But broadly speaking, it would seem that in many races, Republicans will be able to exploit the sweetheart deal Democrats are cutting for their union allies to expedite passage of a highly unpopular piece of legislation.